perf: avoid unnecessary dict allocation in execute_async custom_payload

Remove the unconditional empty dict allocation on the hot path of
execute_async(). Previously, every call allocated {} even when no
custom_payload or execute_as was provided (>99% of calls). The empty
dict was passed to update_custom_payload() which short-circuited on
'if other:' since {} is falsy.

Now custom_payload remains None by default and a dict is only allocated
when execute_as is truthy (the rare DSE proxy-execute case).
update_custom_payload(None) correctly short-circuits at 'if other:'.

Benchmark (5M calls, common case - no payload, no execute_as):
  Before: 280 ns/call
  After:  88 ns/call
  Speedup: 3.19x

Author: mykaul