Where does the documentation say that it reads the ProcessInformation struct? The documentation says:

lpProcessInformation [out]
A pointer to a PROCESS_INFORMATION structure that receives identification information about the new process.
This gives me the impression that there will only be information written to the ProcessInformation struct about a newly created process. Thus leaving garbage values in the ProcessInformation struct will result in the same behaviour as setting everything to zero before passing it to CreateProcess.