Fix warning from passing null to explode

...using is_string() conditional instead of ! is_array() && ! is_null().

Author: steven-fox

src/QueryBuilderRequest.php

@@ -38,7 +38,7 @@ public function includes(): Collection
 
         $includeParts = $this->getRequestData($includeParameterName);
 
-        if (! is_array($includeParts) && ! is_null($includeParts)) {
+        if (is_string($includeParts)) {
             $includeParts = explode(static::getIncludesArrayValueDelimiter(), $includeParts);
         }