C++ std::strings are “counted” strings – i.e., their length is stored as an integer, and they can contain any character. When you replace the third character with a nothing special happens – it’s printed as if it was any other character (in particular, your console simply ignores it).
In the last line, instead, you are printing a C string, whose end is determined by the first that is found. In such a case, cout goes on printing characters until it finds a , which, in your case, is after the third h.
C++ has two string types:
The built-in C-style null-terminated strings which are really just byte arrays and the C++ standard library std::string class which is not null terminated.
Printing a null-terminated string prints everything up until the first null character. Printing a std::string prints the whole string, regardless of null characters in its middle.