fix NullPointerException in Document.getRevision()
fix NullPointerException in Document.getRevision()

If you try to get a specific revision using Document.getRevision, but
the current revision has never been retrieved (and so currentRevision is
null), a NullPointerException is thrown.

mariosotil/couchbase-lite-java-core,couchbase/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,gotmyjobs/couchbase-lite-java-core,deleet/couchbase-lite-java-core,4u7/couchbase-lite-java-core,netsense-sas/couchbase-lite-java-core,mariosotil/couchbase-lite-java-core,Spotme/couchbase-lite-java-core