change lower bound of Option.orNull to nullable

[olhotak]

This PR changes the signature of Option.orNull from

  @inline final def orNull[A1 >: A](implicit ev: Null <:< A1): A1 = this getOrElse ev(null)

to

  @inline final def orNull[A1 >: A | Null](implicit ev: Null <:< A1): A1 = this getOrElse ev(null)

Under -Yexplicit-nulls, type inference sometimes infers A1 to be a non-null type, leading to a failure to find the implicit ev. Changing the lower bound guides type inference to infer a nullable type.

Without -Yexplicit-nulls, the lower bounds A and A | Null are equivalent.

How much have you relied on LLM-based tools in this contribution?

No.

How was the solution tested?

Added test option-ornull minimized from akka-management.

Additional notes

This may have to wait until a version when we can start making changes to standard library signatures.