[SPARK-18815][SQL] Fix NPE when collecting column stats for string/binary...

[SPARK-18815][SQL] Fix NPE when collecting column stats for string/binary column having only null values

## What changes were proposed in this pull request?

During column stats collection, average and max length will be null if a column of string/binary type has only null values. To fix this, I use default size when avg/max length is null.

## How was this patch tested?

Add a test for handling null columns

Author: wangzhenhua <wangzhenhua@huawei.com>

Closes #16243 from wzhfy/nullStats.