Bugfix immutable result set

[AlexDo1]

Quick bugfix: ImmutableResultSets now always contain the input entry if the entry has no associated_groups.

[codecov[bot]]

Codecov Report

Merging #182 (846ea07) into master (e95c325) will decrease coverage by 2.32%. The diff coverage is 0.78%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   62.68%   60.36%   -2.33%     
==========================================
  Files          68       70       +2     
  Lines        3256     3383     +127     
==========================================
+ Hits         2041     2042       +1     
- Misses       1215     1341     +126     

Flag	Coverage Δ
e2e	60.36% <0.78%> (-2.33%)	:arrow_down:

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Impacted Files	Coverage Δ
metacatalog/ext/export_standard/__init__.py	0.00% <0.00%> (ø)
metacatalog/ext/export_standard/extension.py	0.00% <0.00%> (ø)
metacatalog/util/results.py	75.42% <50.00%> (-0.22%)	:arrow_down:

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[AlexDo1]

The plan was to create the branch from master, this PR should not contain the ISO19115 commits, I can do the PR again (@mmaelicke).