Use boolean array for nulls parameters

Hi,

When I review the [1], I find that the tuple's nulls array use char type.
However there are many places use boolean array to repsent the nulls array,
so I think we can replace the char type nulls array to boolean type.  This
change will break the SPI_xxx API, I'm not sure whether this chagnges cause
other problems or not.  Any thought?

[1] - https://www.postgresql.org/message-id/flat/CA+HiwqGkfJfYdeq5vHPh6eqPKjSbfpDDY+j-kXYFePQedtSLeg@mail.gmail.com

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.