Check for non-empty length of previous metadata

[dmjb]

If there is no prior alert state for a rule/entity pair, an empty db result struct is passed around in the evaluation context. Due to a change from an inner join to a left join, the field's type changed from a pointer to a byte slice to just a byte slice. In the empty case, this means that the alert metadata is represented by an empty byte slice instead of a nil. Change the code to check for empty length in addition to nil-ness.

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