Modify PageIsEmpty and PageGetMaxOffsetNumber macros to behave sanely

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.