[SPARK-10835][ML] Word2Vec should accept non-null string array, in addition to...

[SPARK-10835][ML] Word2Vec should accept non-null string array, in addition to existing null string array

## What changes were proposed in this pull request?

To match Tokenizer and for compatibility with Word2Vec, output a nullable string array type in NGram

## How was this patch tested?

Jenkins tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #15179 from srowen/SPARK-10835.