size of a SQL NULL column. (Bug #40369)

When MySQL Bug #20877 was fixed in r834, this function was
accidentally modified to return 0 or 1. Apparently, the only impact of
this bug is that fixed-length columns cannot be updated in-place from
or to SQL NULL, even in ROW_FORMAT=REDUNDANT.  After this fix,
fixed-length columns in ROW_FORMAT=REDUNDANT will have a constant
storage size as they should, no matter if NULL or non-NULL.  The bug
caused fixed-length NULL columns to occupy 1 byte.

rb://37 approved by Heikki over IM.