Fix cctz::format()/parse() to treat NULs like any other chars

The previous commit (PR google#337) was couched in terms of `parse()`
rejecting input strings containing NULs.  Instead, we should
treat NULs like other characters by allowing them in both the
format and input strings.

Similarly, `format()` should allow NULs in its format string.

Also:
 - Simplify `FormatTM()` calls by handling an empty format
   string internally, rather than by guarding each invocation.
 - Utilize the `std::string` iterator-pair constructor and
   `append()`.

Author: devbww