MM-9983 Requiring SiteURL to be set. (#8769)

* Requiring SiteURL to be set.

* Modifying to make tests pass.

* Fixing test.

model/config_test.go

@@ -82,6 +82,13 @@ func TestConfigDefaultFileSettingsS3SSE(t *testing.T) {
 	}
 }
 
+func TestConfigDefaultSiteURL(t *testing.T) {
+	c1 := Config{}
+	c1.SetDefaults()
+
+	assert.Equal(t, "", *c1.ServiceSettings.SiteURL, "SiteURL should be empty by default.")
+}
+
 func TestConfigDefaultServiceSettingsExperimentalGroupUnreadChannels(t *testing.T) {
 	c1 := Config{}
 	c1.SetDefaults()