Fix strlen null error in PHP 8.3 (#14)

* Check for null in resource ID before trying to get the string length.
* Removing workflow (no phpunit tests).

Author: brianhogg

.changelogs/fix_php-83-warning.yml

@@ -0,0 +1,7 @@
+significance: patch
+type: fixed
+links:
+  - "#13"
+attributions:
+  - "@jv-mtrz"
+entry: Fixing null strlen error in PHP 8.3.

.github/workflows/test-phpunit.yml

@@ -145,7 +145,7 @@ private static function get_supported_commands( $route, $route_data ) {
 			$parsed_args   = preg_match_all( '#\([^\)]+\)#', $route, $matches );
 			$resource_id   = ! empty( $matches[0] ) ? array_pop( $matches[0] ) : null;
 			$trimmed_route = rtrim( $route );
-			$is_singular   = $resource_id === substr( $trimmed_route, - strlen( $resource_id ) );
+			$is_singular   = $resource_id === substr( $trimmed_route, - strlen( $resource_id ?? '' ) );
 
 			// List a collection
 			if ( array( 'GET' ) == $endpoint['methods']