fix(gatsby): Improve warning for built-in GraphQL type overrides (#21899)

* fix(gatsby): improve warning for built-in types overrides

* Fixed linting error

Author: vladar

packages/gatsby/src/schema/schema.js

@@ -373,13 +373,19 @@ const mergeTypes = ({
     addExtensions({ schemaComposer, typeComposer, plugin, createdFrom })
 
     return true
-  } else {
+  } else if (typeOwner) {
     report.warn(
       `Plugin \`${plugin.name}\` tried to define the GraphQL type ` +
         `\`${typeComposer.getTypeName()}\`, which has already been defined ` +
         `by the plugin \`${typeOwner}\`.`
     )
     return false
+  } else {
+    report.warn(
+      `Plugin \`${plugin.name}\` tried to define built-in Gatsby GraphQL type ` +
+        `\`${typeComposer.getTypeName()}\``
+    )
+    return false
   }
 }