Fix mutating path of URL without authority (idempotency, empty path segments)

[theskim]

From Issue #984

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
url/src/path_segments.rs	88.88%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #996   +/-   ##
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  Coverage        ?   80.53%           
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  Files           ?       24           
  Lines           ?     4279           
  Branches        ?        0           
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  Hits            ?     3446           
  Misses          ?      833           
  Partials        ?        0           

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[valenting]

If I'm not mistaken, these failing tests should also be made to pass as part of this test

rust-url/url/tests/expected_failures.txt

Lines 39 to 40 in da64903

	<non-spec:/.//p> set hostname to <h>
	<non-spec:/.//p> set hostname to <>

That's about mutating the host, not the path, but I think the behaviour is closely related to this one.