polish: remove unnecessary Maybe (graphql#3683)

Author: yaacovCR

src/type/validate.ts

@@ -647,7 +647,7 @@ function getAllImplementsInterfaceNodes(
 function getUnionMemberTypeNodes(
   union: GraphQLUnionType,
   typeName: string,
-): Maybe<ReadonlyArray<NamedTypeNode>> {
+): ReadonlyArray<NamedTypeNode> {
   const { astNode, extensionASTNodes } = union;
   const nodes: ReadonlyArray<UnionTypeDefinitionNode | UnionTypeExtensionNode> =
     astNode != null ? [astNode, ...extensionASTNodes] : extensionASTNodes;