Fix handling of empty files in frontends (fix #186)

[reuterbal]

No description provided.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/187/index.html

[codecov]

Codecov Report

Merging #187 (443da5b) into main (df3f99a) will increase coverage by 0.02%.
Report is 25 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   92.12%   92.14%   +0.02%     
==========================================
  Files          91       92       +1     
  Lines       16718    16761      +43     
==========================================
+ Hits        15402    15445      +43     
  Misses       1316     1316              

Flag	Coverage Δ
lint_rules	96.22% <ø> (ø)
loki	92.11% <100.00%> (+0.02%)	⬆️
transformations	91.40% <ø> (ø)

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Files	Coverage Δ
loki/backend/fgen.py	97.11% <100.00%> (ø)
loki/frontend/fparser.py	93.07% <100.00%> (ø)
loki/frontend/source.py	96.49% <100.00%> (+0.04%)	⬆️

... and 1 file with indirect coverage changes

[mlange05]

Uncontroversial, but needs a rebase now. GTG from my side.

Edit: No rebase needed now, I think. GTG.