config: remove pycompat.bytestr() for defaultvalue
This is a follow-up patch to 51a2e3102db2. This removes
`pycompat.bytestr` to preserve `None` in `commands.config()`.
Differential Revision: https://phab.mercurial-scm.org/D6712
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\nOK.