When stubbing with argument matchers, the following
scenario would cause a NullPointerException:

```
whenever(mock.doSomething(argThat { })).thenReturn("")
whenever(mock.doSomething(argThat { })).thenReturn("")
```

The second `argThat` invocation returns a `null` value,
which gets propagated into the function passed to the
first `argThat` invocation.

This commit filters `null` values to always evaluate to
`false` when provided to an argument matcher.

Handle `null` in argument matchers.