ScalarFunctionExpr does not preserve the nullable flag on serialization roundtrip

[ccciudatu]

Describe the bug

A physical plan that contains scalar functions will always set the nullable flag to true after deserialization.

To Reproduce

The coalesce function results in a non-nullable field. However, if the plan is serialized and deserialized, the same field will be nullable.
A test to showcase exactly this will be included in the upcoming PR.

Expected behavior

The plan schema should stay the same after a roundtrip.

Additional context

No response