fix(ffi): fix Rust dangling pointer (#5230)
### Description
Fixes #5202
The underlying issue is how protobuf encodes a message with a single
optional field. If that single field is missing the message gets encoded
as a zero length string of bytes. This presents a problem with our usage
of `Vec::as_mut_ptr` which: ["Returns a dangling raw pointer valid for
zero sized reads if the vector didn’t
allocate."](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.as_mut_ptr)
This caused us to send a buffer of length 0, but a dangling data pointer
instead of a null pointer. This wasn't a consistent failure as Go only
checks the validity of all the pointer in the stack under certain
conditions. Specifically when growing/shrinking the stack and needing to
[adjust pointers in a
frame](https://go.googlesource.com/go/+/refs/heads/master/src/runtime/stack.go#626).
### Testing Instructions
Added unit tests that deal with zero length buffers:
- Rust side ensure that if a protobuf message is zero length we return a
buffer with length `0` and the data pointer is the null pointer
- Go side the ensures that a buffer with `0` length and a null data
pointer gets converted to an empty byte slice
---------
Co-authored-by: Chris Olszewski <Chris Olszewski>
