chore(isMailtoURI): remove unnecessary default to

[WikiRik]

Line 44 is only partially covered before this change. This PR hopes to change that so the newly open PRs don't fail codecov/project. Splitting a query always returned at least 1, so the case where the first element (to) was empty couldn't get hit by tests (at least not in our current test setup where we only test strings).

Checklist

• [X] PR contains only changes related; no stray files, etc.
• [X] README updated (where applicable)
• [X] Tests written (where applicable)
• [X] References provided in PR (where applicable)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b958bd7) 99.95% compared to head (15f83b0) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #2341      +/-   ##
===========================================
+ Coverage   99.95%   100.00%   +0.04%     
===========================================
  Files         107       107              
  Lines        2454      2449       -5     
  Branches      619       619              
===========================================
- Hits         2453      2449       -4     
+ Partials        1         0       -1     

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