[SPARK-15707][SQL] Make Code Neat - Use map instead of if check.

## What changes were proposed in this pull request?
In forType function of object RandomDataGenerator, the code following:
if (maybeSqlTypeGenerator.isDefined){
  ....
  Some(generator)
} else{
 None
}
will be changed. Instead, maybeSqlTypeGenerator.map will be used.

## How was this patch tested?
All of the current unit tests passed.

Author: Weiqing Yang <yangweiqing001@gmail.com>

Closes #13448 from Sherry302/master.