[SPARK-20164][SQL] AnalysisException not tolerant of null query plan.

## What changes were proposed in this pull request?

The query plan in an `AnalysisException` may be `null` when an `AnalysisException` object is serialized and then deserialized, since `plan` is marked `transient`. Or when someone throws an `AnalysisException` with a null query plan (which should not happen).
`def getMessage` is not tolerant of this and throws a `NullPointerException`, leading to loss of information about the original exception.
The fix is to add a `null` check in `getMessage`.

## How was this patch tested?

- Unit test

Author: Kunal Khamar <kkhamar@outlook.com>

Closes #17486 from kunalkhamar/spark-20164.