Fix error where zero sources are up over the horizon

[tyler-a-cox]

finufft complains if the source vector has zero elements, which can happen if all sources are below the horizon. This PR skips finufft if the number of sources above the horizon is zero.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 92.91%. Comparing base (5c189c9) to head (c8ebb09).

Files	Patch %	Lines
fftvis/simulate.py	0.00%	2 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   95.93%   92.91%   -3.03%     
==========================================
  Files           4        4              
  Lines         123      127       +4     
  Branches       33       35       +2     
==========================================
  Hits          118      118              
- Misses          3        5       +2     
- Partials        2        4       +2     

Flag	Coverage Δ
unittests	92.91% <0.00%> (-3.03%)	⬇️

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