Prevent empty query var from causing false positive endpoint detection

[westonruter]

Amends #5558. See #2204.

Thanks to @milindmore22 for pointing this out, there's a bug with in how the AMP endpoints are detected. In particular, when a non-AMP URL is request with an empty query var, e.g. ?foo=, the result is the URL erroneously being identified as an AMP one. The reason for this is that I was removing the endpoint from the URL and then checking of the endpoint-removed URL was not equal to the original URL as a shortcut to identify an AMP URL. This, however, doesn't work because remove_query_var() doesn't preserve the distinction between a query var with an empty value and one with no value:

wp> remove_query_arg( 'foo', '?bar=' )
=> string(4) "?bar"
wp> remove_query_arg( 'foo', '?bar' )
=> string(4) "?bar"

Notice how ?bar= and ?bar both end up the same as ?bar when being passed into remove_query_arg().

So the fix is to check if the URL has the endpoint first, and then if it has it, proceed with removing it. This avoids the false positive.

[codecov]

Codecov Report

Merging #5804 (6770c1b) into develop (4890db0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             develop    #5804   +/-   ##
==========================================
  Coverage      75.12%   75.12%           
  Complexity      5645     5645           
==========================================
  Files            210      210           
  Lines          16954    16955    +1     
==========================================
+ Hits           12736    12737    +1     
  Misses          4218     4218           

Flag	Coverage Δ	Complexity Δ
javascript	75.89% <ø> (ø)	0.00 <ø> (ø)
php	75.09% <100.00%> (+<0.01%)	0.00 <0.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
src/PairedRouting.php	96.01% <100.00%> (+0.01%)	111.00 <0.00> (ø)

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[westonruter]

QA Passed

Going to /sample-page/?foo=1 does not serve an AMP page whereas going to /sample-page/?amp=1 does.