fix(gatsby-source-wordpress):issue #29535 not finished createSchemaCu… (

#29554) (#29712)

Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>
(cherry picked from commit 308d418)

Co-authored-by: liwenzhi <lwz7512@gmail.com>
Co-authored-by: Matt Kane <matt@gatsbyjs.com>

packages/gatsby-source-wordpress/src/steps/create-schema-customization/build-types.js

@@ -134,7 +134,10 @@ const objectType = typeBuilderApi => {
   })
 
   // if all child fields are excluded, this type shouldn't exist.
-  if (!Object.keys(transformedFields).length) {
+  // check null first, otherwise cause:
+  // TypeError: Cannot convert undefined or null to object at Function.keys (<anonymous>)
+  // Also cause wordpress blog site build failure in createSchemaCustomization step
+  if (!transformedFields || !Object.keys(transformedFields).length) {
     return
   }