safe_url_string(): add the path for problematic scenarios

[AdrianAtZyte]

URLs without a path but with a query have shown to be problematic, have safe_url_string() force the URL path to prevent such issues.

Fixes scrapy/scrapy#1133, closes scrapy/scrapy#3494, fixes scrapy/scrapy#6574.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.47%. Comparing base (6704653) to head (b93e1de).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #297   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files           9        9           
  Lines         853      855    +2     
  Branches      177      178    +1     
=======================================
+ Hits          840      842    +2     
  Misses          5        5           
  Partials        8        8           

Files with missing lines	Coverage Δ
w3lib/url.py	99.53% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 45 untouched benchmarks

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_{Comparing AdrianAtZyte:url-path (b93e1de) with master (6704653)}