Port `IsNullExpr` to use `try_to_proto` / `try_from_proto`

[adriangb]

Sub-issue of #22418. Port IsNullExpr to use the try_to_proto / try_from_proto hooks added in #21929.

Replace the IsNullExpr arm in datafusion/proto/src/physical_plan/{to_proto,from_proto}.rs with try_to_proto / try_from_proto impls next to IsNullExpr. See the Column / BinaryExpr migrations in #21929 for reference.

[chakkk309]

take