Fix undefined array key 'host' in canonical.php

[geekofshire]

This PR addresses PHP warnings in the WordPress canonical.php file related to undefined host and scheme array keys, particularly in environments using HTTP/3 (QUIC) with Nginx, where these headers may be inconsistently set.

To resolve these warnings, fallback values have been introduced for the host and scheme keys as follows:

• host: Defaults to an empty string ('') if not set. This prevents undefined key errors while allowing flexibility in URL handling logic.
• scheme: Defaults to 'http' if not set. This ensures URLs are built with a valid protocol if the scheme is missing.
  These changes prevent PHP warnings while maintaining backward compatibility across HTTP/1.1, HTTP/2, and HTTP/3 requests.

Trac ticket: https://core.trac.wordpress.org/ticket/62334

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