refactor: Port IsNotNullExpr proto serialization hooks

[chakkk309]

Which issue does this PR close?

• Closes Port IsNotNullExpr to use try_to_proto / try_from_proto #22424.

Rationale for this change

This is part of #22418, which migrates built-in PhysicalExpr implementations away from the central protobuf serialization / deserialization chains.

IsNotNullExpr can now own its protobuf serialization through PhysicalExpr::try_to_proto and its deserialization through IsNotNullExpr::try_from_proto, matching the pattern introduced for Column and BinaryExpr.

What changes are included in this PR?

• Adds PhysicalExpr::try_to_proto support for IsNotNullExpr.
• Adds IsNotNullExpr::try_from_proto.
• Wires IsNotNullExpr deserialization in from_proto.rs through the new hook.
• Removes the old IsNotNullExpr serialization branch from the central to_proto.rs downcast chain.
• Adds direct proto hook tests for:
  • successful IsNotNullExpr roundtrip
  • wrong protobuf expression variant
  • missing required child expression

Are these changes tested?

Yes.

cargo fmt --all --check
git diff --check
cargo test -p datafusion-physical-expr --features proto is_not_null_
cargo clippy -p datafusion-proto --tests -- -D warnings
cargo test -p datafusion-proto --test proto_integration 

Are there any user-facing changes?

No.