Remove old pre-C++11 non-portable helper code.

With C++11, vector::data is guaranteed to return a valid pointer p regardless of the range size such that [p, p + size) is a valid range, and basic_string always contains a null terminator.

Current consuming code never checks for the nullness (which already isn't guaranteed for vector by the current implementation) and passes the size separately to the eventual consumer.

PiperOrigin-RevId: 177104032