Refine `TransactionalOperator.executeAndAwait` nullability

[sdeleuze]

As pointed out in 09c64ae#r881209396, currently TransactionalOperator.executeAndAwait has a rigid null-safety handling and hardcode non-null for the input parameter type and nullable for the return value.

Kotlin allows to deal with both non-null and nullable depending on the nullability of the lambda, we should take advantage of that.