Minimze empty array allocations via centralized helper for pre-net46

[stevenaw]

net462 + netstandard2 define a static Array.Empty<T>() helper which returns a cached empty array. There's a few places in the code where we #ifdef this in where it's easy but there's other places where this approach would be cumbersome. It might help to both cleanup the code + give a minor perf boost (via reduced allocations) by creating a helper to handle this.

This would be most useful for v3.13.x but could also have a place in v4 if we continue to target net45 there (#4036). If net45 support is dropped in v4, then we should instead update all callsites to just call Array.Empty<T> directly instead

[mikkelbu]

@stevenaw It looks like #4042 didn't close this issue - even though it had a Fixes #4041 ??? - (also they are not linked by GH).

EDIT: I guess is it since we are not merging to master, but to a branch.

I've linked this PR now and will close this issue. Please reopen if you disagree.

[mikkelbu]

Closed by #4042

[stevenaw]

Hmm thanks for catching this @mikkelbu ! I wonder if the auto closing function only works for merging to main