fix(core): RetrofitError thrown on login (#1737)

This fixes a regression from when PermissionService was converted from
Groovy to Java. Ideally, we'd remove the extra parameter from
FiatService directly, but that requires patching Fiat first, something
that'll be easier to do when we migrate to the monorepo.

gate-core/src/main/java/com/netflix/spinnaker/gate/services/PermissionService.java

@@ -77,7 +77,9 @@ public void login(final String userId) {
       try {
         AuthenticatedRequest.allowAnonymous(
             () -> {
-              getFiatServiceForLogin().loginUser(userId, null);
+              // TODO(jvz): FiatService::loginUser should have only one parameter as Retrofit no
+              // longer requires this body parameter
+              getFiatServiceForLogin().loginUser(userId, "");
               permissionEvaluator.invalidatePermission(userId);
               return null;
             });