IXR: check if host key exists before accessing it in IXR_Client

[bluefuton]

Summary

• IXR_Client accesses $bits['host'] from parse_url() without checking if the key exists, which can trigger an undefined index warning for URLs without a host component (e.g. relative paths).
• Uses the null coalescing operator (??) to fall back to an empty string, consistent with how port and path are already handled on the adjacent lines.
• Adds a unit test to verify no error is triggered when the host is missing.

Trac ticket

https://core.trac.wordpress.org/ticket/64635

Test plan

• Verify IXR_Client no longer triggers an undefined index warning when instantiated with a URL lacking a host component
• Run npm run test:php -- --filter test_ixr_client_does_not_error_when_host_is_ missing and confirm the test passes

[github-actions]

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Props bluefuton, westonruter.

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[github-actions]

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[westonruter]

Should the same fix be applied to WP_HTTP_IXR_Client?

wordpress-develop/src/wp-includes/class-wp-http-ixr-client.php

Lines 26 to 29 in d3b793f

	$this->scheme = $bits['scheme'];
	$this->server = $bits['host'];
	$this->port = $bits['port'] ?? $port;
	$this->path = ! empty( $bits['path'] ) ? $bits['path'] : '/';