fix(translator): check for default FC existence before clearing it on…

[guydc]

What type of PR is this?

What this PR does / why we need it:

• Avoid nil pointer dereference of a listener's default filter chain when it's nil
• Adds unit tests where translation fails and route cleanup is executed on a listener without a default filter chain

Which issue(s) this PR fixes:

Fixes #5119

Release Notes: No

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 66.89%. Comparing base (dac5532) to head (73f5397).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/xds/translator/translator.go	33.33%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5121      +/-   ##
==========================================
- Coverage   66.89%   66.89%   -0.01%     
==========================================
  Files         210      210              
  Lines       32977    32978       +1     
==========================================
- Hits        22061    22059       -2     
  Misses       9583     9583              
- Partials     1333     1336       +3     

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[liorokman]

LGTM