s:parse_vader: return if there are no lines

[blueyed]

Fixes "E121: Undefined variable: fn".

[codecov-io]

Codecov Report

Merging #173 into master will decrease coverage by 0.08%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   83.42%   83.33%   -0.09%     
==========================================
  Files          12       12              
  Lines         742      744       +2     
==========================================
+ Hits          619      620       +1     
- Misses        123      124       +1

Flag	Coverage Δ
#nvim	82.79% <50%> (-0.09%)	⬇️
#vim	83.06% <50%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
autoload/vader/parser.vim	93.33% <50%> (-0.85%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ffa3190...ea03617. Read the comment docs.