perf(ext/ffi): Avoid receiving on FFI async work channel when no UnsafeCallback exists

[aapoalas]

No performance measurements done yet but this makes the FFI event loop middleware avoid

1. a mutable borrow of the OpState,
2. allocating a (presumably empty) vector,
3. trying to receive on the FfiState's async work channel,
   when no FFI workers (callbacks) exist.

When FFI workers do exist this causes an extra immutable borrow. This could maybe be avoided but it would return the vector allocation to FFI-less middleware work.

[mmastrac]

I like where this PR is going... I suggest a new construction for the middleware callback to avoid extra borrows, but I think we also want to include a smallvec there to avoid hitting the heap any more than we have to.

[mmastrac]

LGTM