Replacing require.nil in api layer (#16906)

* Replacing require.nil in api layer

* Fixing some tests

* Fixing tests

* Addressing PR review comments

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/oauth_test.go

@@ -59,7 +59,7 @@ func TestCreateOAuthApp(t *testing.T) {
 	CheckBadRequestStatus(t, resp)
 
 	r, err := Client.DoApiPost("/oauth/apps", "garbage")
-	require.Error(t, err, "expected error from garbage post")
+	require.NotNil(t, err, "expected error from garbage post")
 	assert.Equal(t, http.StatusBadRequest, r.StatusCode)
 
 	Client.Logout()