Fix taxonomy URLs without path component in canonical redirection.

[snehapatil2001]

This PR fixes an issue in the redirect_canonical() function in wp-includes/canonical.php where URLs without a path component would cause PHP warnings.

The fix adds an explicit check for the existence of the 'path' key before trying to access it and provides a fallback empty path when it's missing.

Trac ticket: https://core.trac.wordpress.org/ticket/63252

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