Respect patterns with empty value and equal lookup

[lundberg]

Fixes #205

[codecov-commenter]

Codecov Report

Merging #206 (d0a1750) into master (019a8a8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #206   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         2731      2735    +4     
  Branches       338       338           
=========================================
+ Hits          2731      2735    +4     

Impacted Files	Coverage Δ
respx/patterns.py	100.00% <100.00%> (ø)
tests/test_patterns.py	100.00% <100.00%> (ø)

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