I've worked on a machine with 9-bit bytes (and 81-bit instructions) and others with 6-bit ones - nether has a C compiler
I don't think there is a C++ compiler for the PDP-10. One of the C compiler does have a 36-bit char type.
Basically, loading a 0-bit byte from memory gets you a 0. Depositing a 0-bit byte will not alter memory, but may do an ineffective read-modify-write cycle. Incrementing a 0-bit byte pointer will leave it unchanged.
As I read it, this link may be describing a hypothetical rather than real compiler. But I did not parse that on initial scan of the Google result.