IsEmpty() checks were missing before some SwitchMemtable() invocation…

[pranavshenoy]

Bug: facebook/rocksdb#12179

There was one instance where the IsEmpty() check was missing:

rocksdb/db/db_impl/db_impl_write.cc

Line 1706 in 21d5a8f

status = SwitchMemtable(cfd, write_context);
Having one check inside SwitchMemTable() will ensure there won't be misses in the future.

[ofriedma]

@pranavshenoy thank you for your contribution.
Currently there are some tests that are failing with this code changes, that should be addressed before merging.

4550 - DBTest.DeletingOldWalAfterDrop (Failed)
12804 - DBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(false, false, 1, 0) (Failed)
12805 - DBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(false, true, 1, 0) (Failed)
12806 - DBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(false, true, 1, 1) (Failed)
12807 - DBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(false, false, 2, 0) (Failed)
12808 - DBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(false, true, 2, 0) (Failed)
13294 - StackableDBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(true, true, 1, 0) (Failed)
13295 - StackableDBAsBaseDB/TransactionTest.TwoPhaseEmptyWriteTest/(true, true, 2, 0) (Failed)

[CLAassistant]

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