migrations: dashboard version migration handles nulls

If a dashboard has a null value for updated_by then the migration
for the dashboard version table fails. This change uses coalesce to
set -1 instead of null when inserting into the created_by column
which has a non-null constraint. Fixes #8783.

pkg/services/sqlstore/migrations/dashboard_version_mig.go

@@ -50,7 +50,7 @@ SELECT
 	dashboard.version,
 	dashboard.version,
 	dashboard.updated,
-	dashboard.updated_by,
+	COALESCE(dashboard.updated_by, -1),
 	'',
 	dashboard.data
 FROM dashboard;`