fix: Flatten empty arrays as null in Arr::dot.

[ropi-bc]

Solves #57305.

The current implementation of Arr::dot() doesn't flatten arrays that contain empty arrays. This goes against my intuition, so I'm proposing a change to reflect what I think should happen instead.

Opened the PR preemptively to:

• Hopefully speed up the process
• Serve as a clear description of my expectations

[taylorotwell]

Won't be changing on a patch release.

[ropi-bc]

do you want me to revisit this change later, or what does this mean exactly? @taylorotwell

[cosmastech]

do you want me to revisit this change later, or what does this mean exactly? @taylorotwell

Probably means make the branch against master 👍