Fix PHPStan return.empty errors in core methods

[huzaifaalmesbah]

This PR addresses strict type return.empty violations reported by PHPStan, ensuring methods return their declared or documented types rather than relying on an empty return;.
Details

• src/wp-includes/class-wp-query.php (WP_Query::setup_postdata):
  • Updated the PHPDoc to reflect that the method can return both true (success) and false (failure).
  • Replaced the early empty return; bail-outs with return false; when the post object is invalid or when generate_postdata() fails.
• src/wp-includes/theme.php (_remove_theme_support):
  • Replaced an empty return; inside the custom-header-uploads backward compatibility fallback with return true;. This matches the documented bool return type to indicate the feature support was successfully removed.
    Why this is needed
    Using empty return; statements in methods/functions that have a declared or documented non-void return type causes static analysis tooling (like PHPStan) to flag them as errors. Returning explicit boolean values improves strict typing constraints and code clarity.

Trac ticket: https://core.trac.wordpress.org/ticket/64238

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