This adds a set of helper classes to determine if an agg "has a value".
This is needed because InternalAggs represent "empty" in different
manners according to convention. Some use `NaN`, `+/- Inf`, `0.0`, etc.
A user can pass the Internal agg type to one of these helper methods
and it will report if the agg contains a value or not, which allows the
user to differentiate "empty" from a real `NaN`.
These helpers are best-effort in some cases. For example, several
pipeline aggs share a single return class but use different conventions
to mark "empty", so the helper uses the loosest definition that applies
to all the aggs that use the class.
Sums in particular are unreliable. The InternalSum simply returns 0.0
if the agg is empty (which is correct, no values == sum of zero). But this
also means the helper cannot differentiate from "empty" and `+1 + -1`.
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