Fix deprecation warning in PHP 8.1+

[circlecube]

PHP Deprecated: stripos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /wp-content/plugins/bluehost-wordpress-plugin/vendor/newfold-labs/wp-module-onboarding/includes/RestApi/RestApiFilter.php on line 134

The code in question looks to be:

public static function is_request_from_onboarding_flow( \WP_REST_Request $request ) {
	return false !== stripos( $request->get_header( 'referer' ), 'page=nfd-onboarding' );
}

which could change to something like:

public static function is_request_from_onboarding_flow( \WP_REST_Request $request ) {
	if ( !$request->get_header('referer') ) {
                 return false;
	} else {
		return false !== stripos( $request->get_header( 'referer' ), 'page=nfd-onboarding' );
	}
}

[abhijitb]

@circlecube : I found another warning when running the flow on PHP 8.1+ .. have fixed that too in linked PR.