Allow empty strings in asserts.

[wRAR]

I think these are more correct, see https://github.com/scrapy/cssselect/pull/121/files#r1003007704

[codecov]

Codecov Report

Merging #130 (eec11c3) into master (f1fcf2b) will decrease coverage by 0.46%.
The diff coverage is 95.58%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
- Coverage   96.12%   95.66%   -0.47%     
==========================================
  Files           2        2              
  Lines         878      899      +21     
  Branches      153      160       +7     
==========================================
+ Hits          844      860      +16     
- Misses         18       22       +4     
- Partials       16       17       +1     

Impacted Files	Coverage Δ
cssselect/parser.py	96.26% <93.33%> (-0.80%)	⬇️
cssselect/xpath.py	94.64% <98.80%> (+0.04%)	⬆️

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