[FIX][MLLIB] fix seed handling in Python GMM

If `seed` is `None` on the python side, it will pass in as a `null`. So we should use `java.lang.Long` instead of `Long` to take it.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4349 from mengxr/gmm-fix and squashes the following commits:

3be5926 [Xiangrui Meng] fix seed handling in Python GMM