Trying to remove a non existing element from a map might cause NPE

Calling SegmentWriter.writeMap(base, changes) with changes containing mappings to null (meaning to remove the respective key) can result in a NPE if base doesn't contain that key.

I came across this while working on the PersistedCompactionMap in OAK-2862. I had to add an extra check for above case as otherwise I'd occasionally hit said NPE.

Need yet to extract a proper test case.