* Alerting: Don't stop the migration when alert rule tags are invalid
As we migrate we expect the `alertRuleTags` on a dashboard alert to be a JSON object. However, it seems this is not really validated by Grafana and an user can change the format to something else that the JSON parser is not able to marshal into a `map[string]string`.
Let's do a bit better by "attempting" to parse the tags and if we can't we'll simple return an empty map. The data is still there so if the user wishes they can go back, fix the data and attemp the migration again.
