Patch sky map when catalog has empty partitions

[camposandro]

Patch the sky map routine to handle empty partitions when a histogram order is specified. Closes #450.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.66%. Comparing base (c9779aa) to head (7db6aea).
Report is 74 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #474   +/-   ##
=======================================
  Coverage   97.65%   97.66%           
=======================================
  Files          39       39           
  Lines        1537     1541    +4     
=======================================
+ Hits         1501     1505    +4     
  Misses         36       36           

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[github-actions]

Before [c9779aa]	After [f212908]	Ratio	Benchmark (Parameter)
2.19±0.01s	2.20±0.03s	1.01	benchmarks.time_create_midsize_catalog
18.1±0.2ms	18.0±0.1ms	1	benchmarks.time_box_filter_on_partition
14.7±0s	14.6±0s	1	benchmarks.time_create_large_catalog
108±0.8ms	106±1ms	0.98	benchmarks.time_kdtree_crossmatch
33.1±0.6ms	32.3±0.6ms	0.98	benchmarks.time_polygon_search

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