truly optional config image_modifier

[vuillaut]

I suppose the intent was to have these optional, since a default value was defined.
However, the reading from the config was mandatory, which is a contradiction.

This PR fixes this.

not passing tests

[rlopezcoto]

sorry, I went too fast, it seems that there is a problem with the location of the variables definition and tests are not passing

[codecov]

Codecov Report

Base: 71.65% // Head: 71.67% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (f0be353) compared to base (4f7e9ec).
Patch coverage: 69.23% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
+ Coverage   71.65%   71.67%   +0.01%     
==========================================
  Files         120      120              
  Lines       11301    11303       +2     
==========================================
+ Hits         8098     8101       +3     
+ Misses       3203     3202       -1     

Impacted Files	Coverage Δ
lstchain/scripts/lstchain_dl1ab.py	84.61% <69.23%> (+2.26%)	⬆️
lstchain/scripts/lstchain_data_r0_to_dl1.py	83.52% <0.00%> (-1.00%)	⬇️
...ain/scripts/onsite/onsite_create_drs4_time_file.py	38.46% <0.00%> (-0.61%)	⬇️
...site/onsite_create_calibration_files_with_batch.py	31.13% <0.00%> (-0.30%)	⬇️
...n/scripts/onsite/onsite_create_calibration_file.py	25.78% <0.00%> (ø)
...s/onsite/onsite_create_ffactor_systematics_file.py	32.43% <0.00%> (ø)

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