Return FALSE instead of throwing error for comparisons with empty ranges.

Change range_before, range_after, range_adjacent to return false rather
than throwing an error when one or both input ranges are empty.

The original definition is unnecessarily difficult to use, and also can
result in undesirable planner failures since the planner could try to
compare an empty range to something else while deriving statistical
estimates.  (This was, in fact, the cause of repeatable regression test
failures on buildfarm member jaguar, as well as intermittent failures
elsewhere.)

Also tweak rangetypes regression test to not drop all the objects it
creates, so that the final state of the regression database contains
some rangetype objects for pg_dump testing.

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