bugfix: handle if errors is not an array

closes mercurius-js#1024

check that error.originalError.errors is an array before mapping it.

if not an array, but is defined, try to handle it anyway.

lib/errors.js

@@ -45,8 +45,12 @@ function defaultErrorFormatter (execution, ctx) {
 
     // it handles fastify errors MER_ERR_GQL_VALIDATION
     if (error.originalError?.errors) {
-      // not all errors are `GraphQLError` type, we need to convert them
-      return error.originalError.errors.map(toGraphQLError)
+      // error.originalError.errors is not always an array, we need to check it first
+      if (Array.isArray(error.originalError.errors)) {
+        // not all errors are `GraphQLError` type, we need to convert them
+        return error.originalError.errors.map(toGraphQLError);
+      }
+      return toGraphQLError(error.originalError.errors);
     }
 
     return error