Fix another Uri debug assert

[MihaZupan]

Overlaps special-cases empty spans as not overlapping even if the pointer appears in the middle.
In this case, the LocalPath test exercises the UriFormat.Unescaped code path.

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[Copilot]

Pull request overview

This PR fixes a debug assertion failure that occurs when processing URIs with empty paths in the UriFormat.Unescaped code path. The issue arises because the Overlaps method can incorrectly report that an empty span doesn't overlap with a buffer, even though the operation is valid.

Changes:

• Modified debug assertion in Uri.cs to handle empty spans correctly
• Added test assertions to verify AbsolutePath and LocalPath properties for an FTP URI with a compressed path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.Uri/src/System/Uri.cs	Fixed debug assertion to check if span is empty before calling Overlaps
src/libraries/System.Private.Uri/tests/FunctionalTests/UriRelativeResolutionTest.cs	Added test assertions to exercise the UriFormat.Unescaped code path with an empty path