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

[liancheng]

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.

[SparkQA]

QA tests have started for PR 2325 at commit 9366c44.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2325 at commit 9366c44.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[marmbrus]

Thanks, I've merged this to master.