[SPARK-19496][SQL] to_date udf to return null when input date is invalid

## What changes were proposed in this pull request?

Currently the udf  `to_date` has different return value with an invalid date input.

```
SELECT to_date('2015-07-22', 'yyyy-dd-MM') ->  return `2016-10-07`
SELECT to_date('2014-31-12')    -> return null
```

As discussed in JIRA [SPARK-19496](https://issues.apache.org/jira/browse/SPARK-19496), we should return null in both situations when the input date is invalid

## How was this patch tested?
unit test added

Author: windpiger <songjun@outlook.com>

Closes #16870 from windpiger/to_date.