Properly handle empty dictionary in get_range_spectrogram

[iaraota]

This PR addresses the bug introduced in #403 by acknowledging that globalv.SPECTROGRAMS might not contain the specified key. In such cases, it now correctly returns an empty SpectrogramList(), according to the previously removed code snippet:

return globalv.SPECTROGRAMS.get(key, SpectrogramList())

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 49.54%. Comparing base (720bd0c) to head (6233e7a).
Report is 4 commits behind head on master.

Files	Patch %	Lines
gwsumm/data/range.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
- Coverage   49.64%   49.54%   -0.10%     
==========================================
  Files          60       60              
  Lines        8823     8831       +8     
==========================================
- Hits         4380     4375       -5     
- Misses       4443     4456      +13     

Flag	Coverage Δ
Linux	49.54% <0.00%> (-0.10%)	⬇️
macOS	49.54% <0.00%> (?)
python3.10	49.54% <0.00%> (-0.10%)	⬇️
python3.11	49.54% <0.00%> (-0.10%)	⬇️
python3.9	49.54% <0.00%> (-0.10%)	⬇️

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[eagoetz]

LGTM