Modify PageIsEmpty and PageGetMaxOffsetNumber macros to behave sanely

if presented an uninitialized (all zeroes) page.  The system no longer
crashes hard if an all-zeroes page is present in a relation.  There seem
to be some boundary conditions where a page will be appended to a relation
and zeroed, but its page header is never initialized; until we can track
down and fix all of those, robustness seems like a good idea.
Also, clean up some obsolete and downright wrong comments.