Issue fix directus#847 (directus#857)

src/core/Directus/Util/StringUtils.php

@@ -233,6 +233,12 @@ public static function replacePlaceholder($string, $data = [], $placeHolderForma
             }
         }
 
+        /**
+         * If any variable of the given string have null value as a replacement then the 
+         * result will be 'null'(string). So we need to replace it with blank string.
+         */
+        $string = str_replace("'null'", "''", $string);
+
         // convert all remaining optionals placeholder from {{optional(key)}} to // 'key' => ''
         $pattern = '#^(.*)({{optional\((.*)\)}})(.*\n)?#im';
         $string = preg_replace_callback($pattern, function ($matches) use ($keyName) {