Don't treat empty endpoint links as invalid

[nekr0z]

An empty string is a valid relative URL that points back to the same page where it is. Both HTTP headers and HTML allow relative URLs. When doing endpoint discovery, those are to be treated as valid values and processed.

Since the empty value was used as an inline error signal, I had to introduce a new error value, have functions return it, and fix some tests to expect it where appropriate.

fixes #12

[codecov-commenter]

Codecov Report

Merging #14 (9a70dc2) into main (8052267) will increase coverage by 2.88%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   70.35%   73.23%   +2.88%     
==========================================
  Files           4        4              
  Lines         199      198       -1     
==========================================
+ Hits          140      145       +5     
+ Misses         39       36       -3     
+ Partials       20       17       -3     

Impacted Files	Coverage Δ
third_party/header/header.go	70.70% <ø> (+1.40%)	:arrow_up:
html.go	85.36% <100.00%> (+1.15%)	:arrow_up:
http.go	100.00% <100.00%> (ø)
webmention.go	66.66% <100.00%> (+5.95%)	:arrow_up:

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[willnorris]

This looks great, thanks!