fixes Location#compareTo(Location)

[BookWood7th]

Related Issue

This pull request addresses #3518.

Intended Change

Adds explicit nullsLast comparators using the natural order of URI and Position when comparing locations.
This avoids a NullPointerException, when fileUri field is null in one of the locations being compared

Type of pull request

• Bug fix (non-breaking change which fixes an issue)
• Refactoring (behaviour should not change or only minimally change)
• There are changes to the (Java) code

Ensuring quality

• I made sure that introduced/changed code is well documented (javadoc and inline comments).
• I have tested the feature as follows: KeY test suite

Additional information and contact(s)

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.

[codecov]

Codecov Report

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

Project coverage is 38.17%. Comparing base (4121ecc) to head (3ebff5e).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/main/java/de/uka/ilkd/key/parser/Location.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3519      +/-   ##
============================================
- Coverage     38.17%   38.17%   -0.01%     
+ Complexity    17227    17226       -1     
============================================
  Files          2109     2109              
  Lines        127624   127630       +6     
  Branches      21461    21461              
============================================
- Hits          48724    48723       -1     
- Misses        72909    72917       +8     
+ Partials       5991     5990       -1     

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[BookWood7th]

There appears to be a test that fails unexpectedly sometimes
de.uka.ilkd.key.rule.merge.MergeRuleTests#testLoadClosedGcdProofWithMergePointStatements().
I believe @WolframPfeifer and I witnessed this test failing before when conducting windows unit tests on #3514, then not failing when running them again.