Fix false positive in if_switch_linter() for empty strings

[emmanuel-ferdman]

PR Summay

if_switch_linter() was flagging if-else chains that compare against empty strings ("" or ''), but we can't actually use empty strings as switch() case names as R throws an error. This PR skips those cases so the linter only suggests switch() when it's actually valid.

Fixes #2835

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.23%. Comparing base (f185cf1) to head (f0f8e57).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3018   +/-   ##
=======================================
  Coverage   99.23%   99.23%           
=======================================
  Files         128      128           
  Lines        7293     7317   +24     
=======================================
+ Hits         7237     7261   +24     
  Misses         56       56           

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[MichaelChirico]

Looking great! I found the original test suite a bit lacking so added a few more tests too as better assurance against backsliding elsewhere from the refactor.

Explaining my tweaks to the helper names:

• I think it's good to emphasize this is specifically an if_else chain
• Hadley influenced me to dislike get_ in function names. I tend to agree it's usually redundant.
• var doesn't precisely match the case like if (foo(x) == 'a') else if (foo(x) == 'b'), I think expr is better

[emmanuel-ferdman]

@MichaelChirico thanks on all the the guidance on this!