Allow reflection_table.remove_on_experiment_identifiers when len==0

[Baharis]

I encountered what I believe is a bug in flex behavior, and since it stops my jobs, I decided to suggest a fix. This PR is tiny, but it is my first one here, so please let me know if I'm missing anything.

When filtering problematic dataset in cctbx.xfel, I encountered multiple instances of the following error:

File "[...]/cctbx_project/xfel/merging/application/filter/experiment_filter.py", line 50, in remove_experiments
  reflections.remove_on_experiment_identifiers(experiment_ids_to_remove)
File "[...]/dials/src/dials/array_family/flex_ext.py", line 1190, in remove_on_experiment_identifiers
  assert "id" in self

The dataset I'm processing has very few reflections, so the error most likely stems from the fact, that an empty flex.reflection_table() does not have key id. However, all methods utilized within remove_on_experiment_identifiers() can act on empty table. In my understanding, the assert "id" in self statement is supposed to catch tables which have reflections, but the reflections have no id.

This PR suggests to add a conditional statement before the assertion, so that the presence of id keyword is required only if the table has any reflections to begin with. As a result, the method should act as a null-operation for a zero-length table. Alternatively, the method could just return self if len(self)==0, but this would create two return points, which is generally undesirable.

[codecov]

Codecov Report

Merging #2298 (529592f) into main (d99f27f) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

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  Branches     8917     8918       +1     
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- Hits        53963    53961       -2     
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- Partials     2063     2066       +3     

[jbeilstenedmands]

Thanks!