[C++] make_span should have an overload for pointer to vector and return an empty span for nulllptr

[stefan301]

It would be handy if there was a make_span overload accepting a pointer to the Vector (like VectorLength). This overload should do the nullptr check and return an empty span in case of nullptr.

If this makes sense, I could make a PR.

[CasperN]

@vglavnyy

[dbaileychess]

@stefan301 Feel free to make a PR.