refactor: Port IsNullExpr proto serialization hooks

[chakkk309]

Which issue does this PR close?

• Closes Port IsNullExpr to use try_to_proto / try_from_proto #22423.

Rationale for this change

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

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

Are these changes tested?

Yes.

I ran:

cargo test -p datafusion-physical-expr --features proto is_null_
cargo clippy -p datafusion-physical-expr --features proto --tests -- -D warnings
cargo test -p datafusion-proto --test proto_integration
cargo clippy --all-targets --all-features -- -D warnings
cargo fmt --all