core: prevent possible NPE

Coverity scan founded that NPE might be thrown in
ImportCommandBase#logImportEvents in case aliasesValuesMap is null.
Thus, null check is added before using the aliasesValuesMap variable.

Change-Id: I43a287f7245da2a4a0e10a71c58613c10df060f2
Signed-off-by: Arik Hadas <ahadas@redhat.com>