Fix field may be null, avoid throwing NullPointerException(#7619)

[skyguard1]

What is the purpose of the change

Fix field may be null, and NullPointerException will be thrown in FieldUtils
In this issue(#7619 ), this pr is to fix this issue, to avoid throwing NullPointerException in FieldUtils, thanks

Brief changelog

Verifying this change

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [Dubbo-XXX] Fix UnknownException when host config not exist #XXX. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Run mvn clean install -DskipTests=false & mvn clean test-compile failsafe:integration-test to make sure unit-test and integration-test pass.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-commenter]

Codecov Report

Merging #7620 (6249f6e) into master (f8e25ec) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #7620   +/-   ##
=========================================
  Coverage     59.11%   59.12%           
- Complexity      528      530    +2     
=========================================
  Files          1076     1076           
  Lines         43443    43445    +2     
  Branches       6346     6347    +1     
=========================================
+ Hits          25681    25686    +5     
- Misses        14917    14926    +9     
+ Partials       2845     2833   -12     

Impacted Files	Coverage Δ	Complexity Δ
...java/org/apache/dubbo/common/utils/FieldUtils.java	84.84% <0.00%> (-5.48%)	0.00 <0.00> (ø)
...bo/config/event/listener/LoggingEventListener.java	50.00% <0.00%> (-12.50%)	0.00% <0.00%> (ø%)
...in/java/org/apache/dubbo/common/utils/JVMUtil.java	81.13% <0.00%> (-11.33%)	0.00% <0.00%> (ø%)
...o/remoting/transport/ChannelHandlerDispatcher.java	60.00% <0.00%> (-10.00%)	0.00% <0.00%> (ø%)
...pache/dubbo/remoting/transport/AbstractServer.java	53.16% <0.00%> (-3.80%)	0.00% <0.00%> (ø%)
...mmon/serialize/kryo/utils/AbstractKryoFactory.java	75.43% <0.00%> (-3.51%)	0.00% <0.00%> (ø%)
...e/dubbo/registry/integration/RegistryProtocol.java	57.94% <0.00%> (-2.81%)	0.00% <0.00%> (ø%)
...pache/dubbo/registry/support/AbstractRegistry.java	78.14% <0.00%> (-1.49%)	0.00% <0.00%> (ø%)
...pache/dubbo/remoting/telnet/codec/TelnetCodec.java	50.61% <0.00%> (-0.62%)	0.00% <0.00%> (ø%)
...n/java/org/apache/dubbo/common/utils/UrlUtils.java	74.14% <0.00%> (-0.35%)	0.00% <0.00%> (ø%)
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f8e25ec...6249f6e. Read the comment docs.

[AlbumenJ]

LGTM.