Computes manifest-level allow_nil? (result nullability) for aggregates.

Per Ash's aggregate semantics, this is independent of the user-facing include_nil? flag (which controls whether nils contribute to the aggregated set — an input concern). The result nullability is determined by the aggregate's kind:

• :count, :exists, :list — never null. :count returns 0 on an empty relationship, :exists returns false, :list returns [].
• :first, :max, :min, :sum, :avg — null on an empty relationship.
• :custom — preserves the aggregate's own allow_nil? if set; otherwise defaults to true (the safe assumption, since the callback's behavior is unknown).