dials.background with no imageset

[dagewa]

Avoid crash like

dials.background non-existent.expt plot=background.png images=1,600
The following parameters have been modified:

images = 1 600
output {
  plot = "background.png"
}

DIALS (2018) Acta Cryst. D74, 85-97. https://doi.org/10.1107/S2059798317017235
DIALS 3.dev
Traceback (most recent call last):
  File "/home/fcx32934/sw/cctbx/build/../modules/dials/src/dials/command_line/background.py", line 247, in <module>
    run()
  File "/home/fcx32934/sw/cctbx/conda_base/lib/python3.9/contextlib.py", line 79, in inner
    return func(*args, **kwds)
  File "/home/fcx32934/sw/cctbx/build/../modules/dials/src/dials/command_line/background.py", line 149, in run
    if len(imagesets) > 1 or len(images) > 1:
UnboundLocalError: Please report this error to dials-support@lists.sourceforge.net: local variable 'images' referenced before assignment

by checking for an empty experiment list.

[codecov]

Codecov Report

Merging #2163 (b745a21) into main (6cefa4f) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2163      +/-   ##
==========================================
- Coverage   80.59%   80.57%   -0.02%     
==========================================
  Files         585      585              
  Lines       66211    66254      +43     
  Branches     9341     9365      +24     
==========================================
+ Hits        53360    53382      +22     
- Misses      10802    10816      +14     
- Partials     2049     2056       +7