do not check if the host is valid if no host is specified and not require_host

[mortbauer]

Fix the isURL check to not check the host if the host part is empty and require_host is false

This PR fixes isse #1994

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1995 (5d2ff83) into master (37cbd5c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1995   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          103       103           
  Lines         2099      2101    +2     
  Branches       474       475    +1     
=========================================
+ Hits          2099      2101    +2     

Impacted Files	Coverage Δ
src/lib/isURL.js	100.00% <100.00%> (ø)

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