dials.sequence_to_stills: skip images with no reflections

[dagewa]

Spotted during small molecule SerialED processing with @jbeilstenedmands: if an image does not have any reflections on it then we do not want to include that stills experiment in the output. Otherwise, dials.ssx_integrate will fail with:

Attempting to split multi-still reflection table
Sorry: Unequal number of reflection tables and experiments after splitting

[codecov]

Codecov Report

Merging #2554 (b8006b5) into main (3cf885b) will decrease coverage by 0.03%.
Report is 3 commits behind head on main.
The diff coverage is 93.58%.

❗ Current head b8006b5 differs from pull request most recent head 7035e2e. Consider uploading reports for the commit 7035e2e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2554      +/-   ##
==========================================
- Coverage   78.84%   78.82%   -0.03%     
==========================================
  Files         609      609              
  Lines       74659    74729      +70     
  Branches    10623    10637      +14     
==========================================
+ Hits        58868    58908      +40     
- Misses      13617    13644      +27     
- Partials     2174     2177       +3