- [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 DUMP_DIRECTORY = "dump.directory";
    public static final String WAIT_KEY = "wait";
    public static final String CLASSIFIER_KEY = "classifier";
    public static final String VERSION_KEY = "version";
    public static final String REVISION_KEY = "revision";
    public static final String DUBBO_VERSION_KEY = "dubbo";
    public static final String HESSIAN_VERSION_KEY = "hessian.version";
    public static final String DISPATCHER_KEY = "dispatcher";
    public static final String CHANNEL_HANDLER_KEY = "channel.handler";
    public static final String DEFAULT_CHANNEL_HANDLER = "default";
    public static final String SERVICE_DESCIPTOR_KEY = "serviceDescriptor";
    public static final String ANY_VALUE = "*";
    public static final String COMMA_SEPARATOR = ",";
    public static final Pattern COMMA_SPLIT_PATTERN = Pattern
            .compile("\\s*[,]+\\s*");
    public final static String PATH_SEPARATOR = "/";
    public final static String PROTOCOL_SEPARATOR = "://";
    public static final String REGISTRY_SEPARATOR = "|";
    public static final Pattern REGISTRY_SPLIT_PATTERN = Pattern
            .compile("\\s*[|;]+\\s*");
    public static final String SEMICOLON_SEPARATOR = ";";
    public static final Pattern SEMICOLON_SPLIT_PATTERN = Pattern
            .compile("\\s*[;]+\\s*");
    public static final String CONNECT_QUEUE_CAPACITY = "connect.queue.capacity";
    public static final String CONNECT_QUEUE_WARNING_SIZE = "connect.queue.warning.size";
    public static final int DEFAULT_CONNECT_QUEUE_WARNING_SIZE = 1000;
    public static final String CHANNEL_ATTRIBUTE_READONLY_KEY = "channel.readonly";
    public static final String CHANNEL_READONLYEVENT_SENT_KEY = "channel.readonly.sent";
    public static final String CHANNEL_SEND_READONLYEVENT_KEY = "channel.readonly.send";
    public static final String COUNT_PROTOCOL = "count";