simplify main class configuration,fix native image build error when mainClass not set

[FoghostCn]

fix native image build error when mainClass is not set

What is the purpose of the change

resolve #13365

mainClass config is not used in dubbo-maven-plugin and dubbo-native module,but if not set it will cause wrong arguments index for generate Aot Assets, so add an empty string as a placeholder when mainClass is not set to prevent build error

now if mainClass is not set, native image can also build success

<plugin>
    <groupId>org.apache.dubbo</groupId>
    <artifactId>dubbo-maven-plugin</artifactId>
    <version>${dubbo.version}</version>
    <executions>
        <execution>
            <phase>process-sources</phase>
            <goals>
                <goal>dubbo-process-aot</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Brief changelog

Verifying this change

Checklist

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

0.0% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a53228d) 40.40% compared to head (c3d993a) 40.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##              3.3   #13426      +/-   ##
==========================================
+ Coverage   40.40%   40.42%   +0.01%     
==========================================
  Files        1807     1807              
  Lines       75867    75867              
  Branches    10642    10643       +1     
==========================================
+ Hits        30657    30669      +12     
+ Misses      40911    40891      -20     
- Partials     4299     4307       +8     

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