schema: make user fields non-nullable

They can always be empty. This simplifies mutation, since graphql_client
doesn't have an easy way to conditionally leave out fields (we could do
that with `@include`, but that's one bool per field in addition to the
field, a bit ugly).

schema.graphql

@@ -45,9 +45,9 @@ type Query {
 type User {
   id: String!
   email: String!
-  displayName: String
-  firstName: String
-  lastName: String
+  displayName: String!
+  firstName: String!
+  lastName: String!
   creationDate: DateTimeUtc!
   "The groups to which this user belongs."
   groups: [Group!]!