Fix the SafeMaskMaker

[bkhelifi]

Description
This pull request is associated to the issue #3984. It fixes 2 things as discussed during the last dev call

• return an empty mask when the effective area is null (issue ExcessMapEstimator fails when dataset has no mask #3983)
• propagate correctly the observation in the mask function (issue SafeMaskMaker fails with fixed_offset option #3984)

Dear reviewer
This PR is aimed for the bug release V0.20.2

[codecov]

Codecov Report

Merging #3987 (af5f86b) into master (7b4dab6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3987      +/-   ##
==========================================
- Coverage   94.04%   94.04%   -0.01%     
==========================================
  Files         162      162              
  Lines       21391    21401      +10     
==========================================
+ Hits        20118    20127       +9     
- Misses       1273     1274       +1     

Impacted Files	Coverage Δ
gammapy/makers/safe.py	88.88% <100.00%> (+1.16%)	⬆️
gammapy/modeling/iminuit.py	92.52% <0.00%> (-1.87%)	⬇️
gammapy/astro/darkmatter/spectra.py	98.36% <0.00%> (ø)
gammapy/modeling/fit.py	95.56% <0.00%> (+0.03%)	⬆️
gammapy/maps/region/geom.py	93.21% <0.00%> (+0.10%)	⬆️

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[registerrier]

Thanks @bkhelifi!

Main comment here is that tests should be added to ensure the issues are properly corrected and that future changes do not break this.

[registerrier]

Thanks @bkhelifi