Add null check for CompositeFrame frames

[FiberMan]

This PR adds a null check for the frames field in CompositeFrame to safely handle cases where a CompositeFrame has a null frames property. At Dat.mobility, we frequently parse NeTEx data containing CompositeFrames with empty "frames" fields, which currently causes errors in the parser.

This fix allows us to use the netex-parser-java library as a regular Maven dependency without needing a forked version just to handle this edge case. The change is minimal but improves robustness when dealing with real-world NeTEx data that may not strictly follow the schema.

[vpaturet]

This looks good.
We use prettier for code formatting. Could you please run the unit tests locally in Maven: this will automatically reformat the code (and fix the indentation errors reported by @testower )
You may have to rebase your PR on the latest version of the master branch, we have just fixed an error in the Maven configuration.

[FiberMan]

Fixed formatting. Thanks!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@1201b9e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...entur/netex/loader/parser/NetexDocumentParser.java	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #240   +/-   ##
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  Coverage          ?   89.62%           
  Complexity        ?      235           
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  Files             ?       16           
  Lines             ?      800           
  Branches          ?       60           
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  Hits              ?      717           
  Misses            ?       56           
  Partials          ?       27           

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