wp_is_stream: assert non-falsy $path

[IanDelMar]

If wp_is_stream($path) returns true, $path is a non-falsy-string; however, there is no corresponding narrowing on the false branch.

Given

$scheme_separator = strpos( $path, '://' );

if ( false === $scheme_separator ) {
	// $path isn't a stream.
	return false;
}

from the WP function reference, any falsy string necessarily causes the function to return false.