Fix #4255 (Fix exception on mixed indent description)

[alanmcruickshank]

This resolves #4255 . The problem arises from trying to make a description for a mixed indent, which we don't actually need. This resolves that gracefully by providing a generic "mixed indent" return so that things can complete nicely.

[WittierDinosaur]

Would we not want to fix a mixed indent? You'd expect a mixture of tabs and spaces to be wrong in any scenario - granted that'd be a more involved change

[alanmcruickshank]

Would we not want to fix a mixed indent? You'd expect a mixture of tabs and spaces to be wrong in any scenario - granted that'd be a more involved change

We do - but that's a different rule. This is a rule aimed just at trailing/leading comments and doesn't include the logic to fix the indent. It's a good question on why someone would only want to do one and not the other - but I could imagine someone with a particularly strange indentation setup might want to use us to lint commas but not indents. 🤷

[coveralls]

Pull Request Test Coverage Report for Build 3928982305

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3928979929:	0.0%
Covered Lines:	16961
Relevant Lines:	16961

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💛 - Coveralls

[codecov]

Codecov Report

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

Coverage data is based on head (f69c8eb) compared to base (c24a636).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #4288   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          201       201           
  Lines        15181     15183    +2     
=========================================
+ Hits         15181     15183    +2     

Impacted Files	Coverage Δ
src/sqlfluff/utils/reflow/elements.py	100.00% <100.00%> (ø)

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[WittierDinosaur]

LGTM