main,Fypp: stop applying the line filter if there is no more input

[masatake]

Close #3493.

[codecov[bot]]

Codecov Report

Base: 83.40% // Head: 83.41% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (092da29) compared to base (132379f). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3495   +/-   ##
=======================================
  Coverage   83.40%   83.41%           
=======================================
  Files         220      220           
  Lines       52967    52977   +10     
=======================================
+ Hits        44178    44191   +13     
+ Misses       8789     8786    -3     

Impacted Files	Coverage Δ
parsers/fypp.c	99.42% <ø> (ø)
main/promise.c	100.00% <100.00%> (+2.38%)	:arrow_up:
main/read.c	95.52% <100.00%> (+0.05%)	:arrow_up:

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