ctdb-common: Fix empty value handling in conf

ctdb-common: Fix empty value handling in conf

In tini, allow_empty_value=false causes the parser to ignore the lines
without '=' sign, but lines with nothing after '=' sign are allowed and
cause empty string ("") to be passed as a value.

This is counter-intuitive, so conf requires special handling for empty
values (which are treated as invalid).

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>