- [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 DEFAULT_TRANSPORTER = "netty";
    public static final String DEFAULT_REMOTING_SERVER = "netty";
    public static final String DEFAULT_REMOTING_CLIENT = "netty";
    public static final String DEFAULT_REMOTING_CODEC = "dubbo";
    public static final String DEFAULT_REMOTING_SERIALIZATION = "hessian2";
    public static final String DEFAULT_HTTP_SERVER = "servlet";
    public static final String DEFAULT_HTTP_CLIENT = "jdk";
    public static final String DEFAULT_HTTP_SERIALIZATION = "json";
    public static final String DEFAULT_CHARSET = "UTF-8";
    public static final int DEFAULT_WEIGHT = 100;
    public static final int DEFAULT_FORKS = 2;
    public static final String DEFAULT_THREAD_NAME = "Dubbo";
    public static final int DEFAULT_CORE_THREADS = 0;
    public static final int DEFAULT_THREADS = 200;
    public static final boolean DEFAULT_KEEP_ALIVE = true;
    public static final int DEFAULT_QUEUES = 0;
    public static final int DEFAULT_ALIVE = 60 * 1000;
    public static final int DEFAULT_CONNECTIONS = 0;
    public static final int DEFAULT_ACCEPTS = 0;
    public static final int DEFAULT_IDLE_TIMEOUT = 600 * 1000;
    public static final int DEFAULT_HEARTBEAT = 60 * 1000;
    public static final int DEFAULT_TIMEOUT = 1000;
    public static final int DEFAULT_CONNECT_TIMEOUT = 3000;
    public static final int DEFAULT_RETRIES = 2;
    public static final int DEFAULT_FAILBACK_TASKS = 100;
    public static final int DEFAULT_FAILBACK_TIMES = 3;
    public static final int MAX_PROXY_COUNT = 65535;
    
    public static final int DEFAULT_BUFFER_SIZE = 8 * 1024;
    public static final Integer DEFAULT_METADATA_REPORT_RETRY_TIMES = 100;