path_is_absolute: narrow return type and assert param type

[IanDelMar]

path_is_absolute():

function path_is_absolute( $path ) {
	if ( wp_is_stream( $path ) && ( is_dir( $path ) || is_file( $path ) ) ) {
		return true;
	}
	if ( realpath( $path ) === $path ) {
		return true;
	}
	if ( strlen( $path ) === 0 || '.' === $path[0] ) {
		return false;
	}
	if ( preg_match( '#^[a-zA-Z]:\\\\#', $path ) ) {
		return true;
	}
	return ( '/' === $path[0] || '\\' === $path[0] );
}

Each if branch as well as the final else branch (return) is false for both, '' and '0'. Therefore, path_is_absolute() always returns false for '' and '0'.

Related: #481