fix SourceNotVisible exception

[b-biswas]

The code crashes when using the catsim catalog because when the source is not visible in one of the bands, render_single() returns None.
and None type has no attribute None.array

[codecov]

Codecov Report

Base: 89.81% // Head: 89.76% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (f6972dd) compared to base (fd2f1aa).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #386      +/-   ##
===========================================
- Coverage    89.81%   89.76%   -0.06%     
===========================================
  Files           11       11              
  Lines         1522     1524       +2     
===========================================
+ Hits          1367     1368       +1     
- Misses         155      156       +1     

Flag	Coverage Δ
unittests	89.76% <75.00%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
btk/draw_blends.py	91.08% <75.00%> (-0.33%)	⬇️

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[ismael-mendoza]

Thanks! Did this happen because you tried to use the bigger catalog? @b-biswas