Null checks for MultiMap methods
client tests edited & generic null check method added
Null checks for MultiMap methods
client tests edited & generic null check method added
Merge branch 'null-check-multimap' of https://github.com/buremba/hazelcast into null-check-multimap
renamed ValidationUtil.checkIfNull method
Merge remote-tracking branch 'hazelcast/master' into null-check-multimap

Conflicts:

hazelcast-client/src/main/java/com/hazelcast/client/proxy/ClientMultiMap
Proxy.java
Merge pull request #3193 from buremba/null-check-multimap

Null checks for MultiMap methods