BUG: Aggregating `duration` in a Dashboard returns an error

[hrsong99]

When creating a Dashboard, there's an issue where aggregating duration (sum, average, min, max) would crash ClickHouse with Illegal type UInt64 of argument of function notEmpty. I'm not exactly sure, but I think the clickhouse conversion function notEmpty() is the problem when using it with a number.

Here's a PR to see if that fixes that: #245