[SPARK-3448][SQL] Check for null in SpecificMutableRow.update

`SpecificMutableRow.update` doesn't check for null, and breaks existing `MutableRow` contract.

The tricky part here is that for performance considerations, the `update` method of all subclasses of `MutableValue` doesn't check for null and sets the null bit to false.

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #2325 from liancheng/check-for-null and squashes the following commits:

9366c44 [Cheng Lian] Check for null in SpecificMutableRow.update
[SPARK-3458] enable python "with" statements for SparkContext

allow for best practice code,

```
try:
  sc = SparkContext()
  app(sc)
finally:
  sc.stop()
```

to be written using a "with" statement,

```
with SparkContext() as sc:
  app(sc)
```

Author: Matthew Farrellee <matt@redhat.com>

Closes #2335 from mattf/SPARK-3458 and squashes the following commits:

5b4e37c [Matthew Farrellee] [SPARK-3458] enable python "with" statements for SparkContext