All CChar does is return the first character of a given expression. Thats it. So CChar prints the character representation of vbNullChar.
Youll probably need byteconverter library and use the ASCII code for Null. However, vbNullChar is just a 0. or ASCII 0x00, so equivalently, your returning the print character of the value 0. in the case of B4X, that would be Chr(0)
when designing/using protocols like this, you really need to understand on a binary level what is going on, which will help you at porting code between languages.