DPL: avoid crash on nullptr in proxy - #10077

@dennisklein @rbx is there any case in which a fairmq::Parts would contain a unique ptr set to nullptr?

You will not have nullptr after receiving into empty Parts.
Parts is a wrapper around std::vector<MessagePtr>.
But you could of course just put nullptr inside yourself. It would also stay there if you receive into this Parts object, since receive appends to the vector.

We also check for nullptrs inside of parts on sending and error if we find any. But we don't put nullptrs inside ourselves, if I remember right.

So the check is not wrong.

this doesn't fix the crash we are seeing (tested in environment id 2b8Hxbczvnd)

Error while checking build/O2/fullCI for 295c4cb at 2022-10-14 18:27:

## sw/BUILD/o2codechecker-latest/log
100% tests passed, 0 tests failed out of 1

Full log here.