fix: only trim XML elements with no inner elements

[cuixq]

This PR fixes two issues when reading/writing Maven manifest:

• when merging parents, if groupId or version is empty, inherit these values from the parent;
• only trim elements with no inner elements, otherwise inner elements will be wiped.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 65.78%. Comparing base (8617d67) to head (6462956). Report is 5 commits behind head on main.

Files	Patch %	Lines
internal/manifest/maven.go	30.00%	4 Missing and 3 partials :warning:
internal/resolution/manifest/maven.go	40.00%	0 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1168 +/- ## ========================================== + Coverage 65.65% 65.78% +0.13% ========================================== Files 164 165 +1 Lines 13750 13898 +148 ========================================== + Hits 9027 9143 +116 - Misses 4229 4251 +22 - Partials 494 504 +10 ```

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[michaelkedar]

• when merging parents, does not check parent.groupId since Maven allows this to happen;

I don't think this is quite the correct behaviour. e.g. if you say your parent's groupId is org.foo but the parent pom says org.bar Maven throws an error.

The problem we are seeing is that the groupId is inherited from the parent if it is missing. So we'd need to add some logic somewhere like

if proj.GroupID == "" {
  proj.GroupID = parent.GroupID
}

[cuixq]

thanks for checking that @michaelkedar !