NPE when PojoUtils transfer Null element in Collection

[HzjNeverStop]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.3-SNAPSHOT
• Operating System version:mac os 10.13.6
• Java version: 1.8.0_171

Steps to reproduce this issue

1. export an dubbo service which has a method with an collection argument

    @Override
    public String testListName(List<String> list) {
        return list.toString();
    }

2. start a telnet invoke or generic call to invoke the service
   dubbo>invoke testListName(["a","b",null])

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

Expected Result

What do you expected from the above steps？

result: "[a, b, null]"

Actual Result

What actually happens?

throw an NPE when pojoUtils invoke method #realize0

If there is an exception, please attach the exception trace:

Failed to invoke method testListName, cause: java.lang.NullPointerException
java.lang.NullPointerException
	at org.apache.dubbo.common.utils.PojoUtils.realize0(PojoUtils.java:369)
	at org.apache.dubbo.common.utils.PojoUtils.realize(PojoUtils.java:205)
	at org.apache.dubbo.common.utils.PojoUtils.realize(PojoUtils.java:95)
	at org.apache.dubbo.rpc.protocol.dubbo.telnet.InvokeTelnetHandler.telnet(InvokeTelnetHandler.java:125)
	at org.apache.dubbo.remoting.telnet.support.TelnetHandlerAdapter.telnet(TelnetHandlerAdapter.java:59)
	at org.apache.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(HeaderExchangeHandler.java:205)
	at org.apache.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:51)
	at org.apache.dubbo.remoting.transport.dispatcher.ChannelEventRunnable.run(ChannelEventRunnable.java:57)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

[CrazyHZM]

Has been fixed.