The reality is that null-terminated strings are dramatically more expensive than strings with a length counter in every regard other than memory usage, and the memory usage overhead from storing a length value is utterly miniscule compared to the actual size of the string. Even if you ignore all the secondary costs that result from the decision to use null-terminated strings, they're just poor engineering. There are far better ways to save a few bytes.

(By secondary costs I mean things like the myriad bugs caused by null-terminated strings, the severe performance penalties involved in copying and manipulating them, the unfortunate implications they have for file formats and network protocols, etc.)