- [x] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.0
* Operating System version: Mac OS
* Java version: Java 8

### Steps to reproduce this issue

run the following UT

```java
    @Test
    public void testConvertPrimitive() {
        Object result = ClassHelper.convertPrimitive(Integer.class, "");
    }
```


Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

No exception is thrown.

### Actual Result

Following exception is thrown:

```
java.lang.NumberFormatException: For input string: ""

	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.base/java.lang.Integer.parseInt(Integer.java:662)
	at java.base/java.lang.Integer.valueOf(Integer.java:983)
	at org.apache.dubbo.common.utils.ClassHelper.convertPrimitive(ClassHelper.java:277)
	at org.apache.dubbo.common.utils.ClassHelperTest.testConvertPrimitive(ClassHelperTest.java:125)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
```

### Improvement

1. Throwing exception is expensive, return null if input String is null or ""
2. Add more test cases to this method.
public static final String ROUTERS_SUFFIX = ".routers";
    public static final String CONFIG_CLUSTER_KEY = "config.cluster";
    public static final String CONFIG_NAMESPACE_KEY = "config.namespace";
    public static final String CONFIG_GROUP_KEY = "config.group";
    public static final String CONFIG_CHECK_KEY = "config.check";
    public static final String CONFIG_CONFIGFILE_KEY = "config.config-file";
    public static final String CONFIG_ENABLE_KEY = "config.highest-priority";
    public static final String CONFIG_TIMEOUT_KEY = "config.timeout";
    public static final String CONFIG_APPNAME_KEY = "config.app-name";
    public static final String DEFAULT_CATEGORY = PROVIDERS_CATEGORY;
    public static final String ENABLED_KEY = "enabled";
    public static final String DISABLED_KEY = "disabled";
    public static final String VALIDATION_KEY = "validation";
    public static final String CACHE_KEY = "cache";
    public static final String DYNAMIC_KEY = "dynamic";
    public static final String DUBBO_PROPERTIES_KEY = "dubbo.properties.file";
    public static final String DEFAULT_DUBBO_PROPERTIES = "dubbo.properties";
    public static final String SENT_KEY = "sent";
    public static final boolean DEFAULT_SENT = false;
    public static final String REGISTRY_PROTOCOL = "registry";
    public static final String $INVOKE = "$invoke";
    public static final String $ECHO = "$echo";
    public static final int DEFAULT_IO_THREADS = Math.min(Runtime.getRuntime().availableProcessors() + 1, 32);
    public static final String DEFAULT_PROXY = "javassist";
    public static final int DEFAULT_PAYLOAD = 8 * 1024 * 1024;                      
    public static final String DEFAULT_CLUSTER = "failover";
    public static final String DEFAULT_DIRECTORY = "dubbo";
    public static final String DEFAULT_LOADBALANCE = "random";
    public static final String DEFAULT_PROTOCOL = "dubbo";
    public static final String DEFAULT_EXCHANGER = "header";