In StreamWriter#write, allow empty strings, disallow null strings

[CDAP-396](https://issues.cask.co/browse/CDAP-396)
In StreamWriter#write, allow empty strings and disallow null strings.
This allows for file-tailer and file-drop-zone to work with files that have empty lines.

I have verified that empty strings can be sent to cdap streams (it increases the stream's event count by 1, and data storage of the stream remains unchanged).