Gracefully handle weighted sampling with no sequences

[victorlin]

Description of proposed changes

When running with --group-by-weights and no sequence records pass earlier filters, get_weighted_group_sizes was called with an empty records_per_group dict. This caused pandas df.apply on an empty DataFrame to return a DataFrame instead of a Series, triggering an unhandled error when setting _augur_filter_input_size.

Exit early in get_weighted_group_sizes when records_per_group is empty so that the command completes and handles empty filter results through --empty-output-reporting.

Related issue(s)

Closes #2032

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.24%. Comparing base (12b55ab) to head (f1e9f15).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/filter/subsample.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2033      +/-   ##
==========================================
- Coverage   73.25%   73.24%   -0.02%     
==========================================
  Files          86       86              
  Lines       10754    10759       +5     
  Branches     2096     2098       +2     
==========================================
+ Hits         7878     7880       +2     
- Misses       2502     2504       +2     
- Partials      374      375       +1     

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