Don't cast between GinNullCategory and bool

The original idea was that we could use an isNull-style bool array
directly as a GinNullCategory array.  However, the existing code already
acknowledges that that doesn't really work, because of the possibility
that bool as currently defined can have arbitrary bit patterns for true
values.  So it has to loop through the nullFlags array to set each bool
value to an acceptable value.  But if we are looping through the whole
array anyway, we might as well build a proper GinNullCategory array
instead and abandon the type casting.  That makes the code much safer in
case bool is ever changed to something else.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/include/access/ginblock.h

@@ -188,8 +188,11 @@ typedef struct
 
 /*
  * Category codes to distinguish placeholder nulls from ordinary NULL keys.
- * Note that the datatype size and the first two code values are chosen to be
- * compatible with the usual usage of bool isNull flags.
+ *
+ * The first two code values were chosen to be compatible with the usual usage
+ * of bool isNull flags.  However, casting between bool and GinNullCategory is
+ * risky because of the possibility of different bit patterns and type sizes,
+ * so it is no longer done.
  *
  * GIN_CAT_EMPTY_QUERY is never stored in the index; and notice that it is
  * chosen to sort before not after regular key values.