Null checks for MultiMap methods

I used NullPointerException because IMap already uses that exception when a user passes null value as parameter. However I'm not sure whether it should be IllegalArgumentException or not.
Since you are already testing a multimap, there is no need to repeat the name in the method.

We try to use the following naming convention as much as possible:

testGet_whenNullKey()

Also make sure that when you add such test, that you group the test of the same method, e.g.

```
//====== get================

void testGet_whenNullKey(){...}
void testGet_whenNullValue(){...}
void testGet_whenNonNullValue(){...}
void testGet_whenLocked(){...}
void testGet_whenProblemDeserializing(){..}
```

As you can see it becomes a lot easier to see which cases have been tested of a particular method