fix: nil: Handle NUL in text files better

NULs in text files usually indicate corruption.  Additionally embedded
NULs can cause text files to be interpreted differently to the way they
appear to be when read by humans.  NUL in text files now returns
token type isc_tokentype_unknown unless escaped or enclosed in double
quotes.

Fixes #5796

Merge branch 'marka-set-specials-zero' into 'main'

See merge request isc-projects/bind9!11669