Fix parse_header_links on empty header

[remram44]

When receiving an empty Link: header, one bogus entry was returned anyway. This breaks some applications.

I agree that web servers shouldn't be sending those but they do so here we are. Also fixes docstring with proper return type.

Fixes halcy/Mastodon.py#74

[codecov-io]

Codecov Report

Merging #4245 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4245      +/-   ##
==========================================
+ Coverage   88.65%   88.66%   +0.01%     
==========================================
  Files          18       18              
  Lines        2018     2021       +3     
==========================================
+ Hits         1789     1792       +3     
  Misses        229      229

Impacted Files	Coverage Δ
requests/utils.py	82.87% <100%> (+0.12%)	⬆️

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[Lukasa]

Thanks for this, this is a good change! Can we get some tests and an entry in HISTORY.rst please?

[remram44]

Sure thing!

[Lukasa]

Beautiful, thank you so much!

[Lukasa]

Congratulations on your first Requests contribution! ✨🍰✨

[remram44]

You guys are very friendly. Thanks for the kind words. I'll try to patch the bogus server as well.