Use Cases with empty cells in the list

For this use case, I would encourage you to use Map and Replace rather than Cases, since you can use multiple rules for the extraction:

Map[Replace[{{___, {}} :> Null, {___, {x_}} :> x}],
 dat1]
(* {Null, 1, 2} *)

As an alternative, you can use Replace with a level spec:

Replace[dat1, {{___, {}} :> Null, {___, {x_}} :> x}, 1]
(* {Null, 1, 2} *)

Here I treat the contents of the empty element as Null and I assume either 1 element in the nested list or no element:

Cases[dat1, {x_ : Null} :> x, 2]
(* returns: {Null, 1, 2} *)

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