PLT-2976 Adding checkout for missing SMTP server when testing email connection (#3115)

* PLT-2976 Adding checkout for missing SMTP server when testing email connection

* Fixing unit test

api/admin.go

@@ -184,6 +184,11 @@ func testEmail(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if len(utils.Cfg.EmailSettings.SMTPServer) == 0 {
+		c.Err = model.NewLocAppError("testEmail", "api.admin.test_email.missing_server", nil, utils.T("api.context.invalid_param.app_error", map[string]interface{}{"Name": "SMTPServer"}))
+		return
+	}
+
 	if result := <-Srv.Store.User().Get(c.Session.UserId); result.Err != nil {
 		c.Err = result.Err
 		return