Improve type stability of basic sources

[oxinabox]

This is pretty minor, as these type instabilities in ifelse are taken care of by union-splitting anyway.
But I did notice them, so thought i would fix

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 54.40%. Comparing base (02a134c) to head (15da497).

Files	Patch %	Lines
src/Blocks/sources.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
- Coverage   61.82%   54.40%   -7.42%     
==========================================
  Files          46       46              
  Lines        1438     1634     +196     
==========================================
  Hits          889      889              
- Misses        549      745     +196     

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[oxinabox]

CodeCov is wrong afaict