Gwb dat check if line is empty

[MFraters]

@danieldouglas92 reported tan issue where the .dat file caused a segmentation fault. It turned out to be due to empty lines. This pull request fixes that issue and add some empty lines to an existing test to make sure we catch it in the future.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.018 ± 0.007 (s=456)	1.017 ± 0.007 (s=431)	-0.2% .. +0.1%
Slab interpolation curved simple none	1.028 ± 0.005 (s=435)	1.028 ± 0.005 (s=443)	-0.2% .. +0.0%
Spherical slab interpolation simple none	1.265 ± 0.010 (s=366)	1.263 ± 0.004 (s=349)	-0.2% .. +0.1%
Slab interpolation simple curved CMS	1.098 ± 0.005 (s=425)	1.099 ± 0.008 (s=397)	-0.1% .. +0.2%
Spherical slab interpolation simple CMS	1.863 ± 0.007 (s=237)	1.863 ± 0.008 (s=248)	-0.1% .. +0.1%
Spherical fault interpolation simple none	1.274 ± 0.007 (s=361)	1.275 ± 0.010 (s=348)	-0.1% .. +0.2%
Cartesian min max surface	2.418 ± 0.016 (s=195)	2.417 ± 0.019 (s=180)	-0.3% .. +0.2%
Spherical min max surface	7.379 ± 0.034 (s=74)	7.372 ± 0.013 (s=51)	-0.3% .. +0.1%

[codecov]

Codecov Report

Merging #519 (e642a04) into main (78f1200) will not change coverage.
Report is 6 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #519   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files          92       92           
  Lines        6279     6279           
=======================================
  Hits         5868     5868           
  Misses        411      411           

Files	Coverage Δ
source/gwb-dat/main.cc	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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