[Execution state] Reduce memory allocs by reusing same immutable empty payload

[fxamacker]

Problem

We're creating a new empty payload (struct with 2 slices) on the heap each time one is needed.

Scenarios that create empty payloads include:

• read() creates new empty payloads when we try to read a non-existent path
• Forest.Proofs() creates new empty payloads to generate proof for non-existent path

And all payloads are deep copied upon read from the trie and update to the trie.

The Proposed Solution

Reuse the same immutable empty payload instead of creating a new one each time.

Updates epic #1744

[fxamacker]

I'm closing this issue because it doesn't seem like it would have enough impact to be worthwhile.