Warn and return empty SpotAttributes when PixelDecoding finds 0 spots

[shanaxel42]

fixes: #1275

Test: Run BaristaSeq with Clip(expand_dynamic_range=False)

[codecov-io]

Codecov Report

Merging #1400 into master will decrease coverage by 1.27%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1400      +/-   ##
==========================================
- Coverage   90.69%   89.41%   -1.28%     
==========================================
  Files         223      152      -71     
  Lines        8520     5536    -2984     
==========================================
- Hits         7727     4950    -2777     
+ Misses        793      586     -207

Impacted Files	Coverage Δ
.../spots/_detect_pixels/combine_adjacent_features.py	85.32% <50%> (-1.35%)	⬇️
starfish/core/image/_filter/element_wise_mult.py	88.57% <0%> (-1.43%)	⬇️
starfish/core/experiment/test/test_experiment.py
starfish/core/recipe/test/test_runnable.py
starfish/core/image/_filter/test/test_reduce.py
starfish/core/imagestack/test/test_export.py
...intensity_table/test/test_synthetic_intensities.py
...rfish/core/intensity_table/test/test_to_mermaid.py
starfish/core/test/factories.py
...tarfish/core/image/_filter/test/test_call_bases.py
... and 64 more

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