Fix swap_lon_axis() breaking when sorting with singleton coords

[tomvothecoder]

Description

• Closes [Bug]: Singletons breaks default sort_ascending=True in swap_lon_axis() #389

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (580f5e9) compared to base (d5d4fd7).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #392   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1243      1245    +2     
=========================================
+ Hits          1243      1245    +2     

Impacted Files	Coverage Δ
xcdat/axis.py	100.00% <100.00%> (ø)

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