Fix nullability annotations in NITableViewModel (#701)
Fix nullability annotations in NITableViewModel (#701)

These were failing with the following error:

```
nimbus/src/models/src/NITableViewModel.m:61:16: error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
  return [self initWithDelegate:nil];
```
jverkoey/nimbus,jverkoey/nimbus,jverkoey/nimbus,jverkoey/nimbus,jverkoey/nimbus,jverkoey/nimbus,jverkoey/nimbus