Bugfix for empty string validation

[thekaranacharya]

• Fixes this [bug] ToxicLanguage doesn't work with streaming and OpenAI  #619
• Was previously always returning None as error for StringSchema's parse() method. This allowed even empty strings/None to fail validators which expect only non-empty strings.
• Solution: Return True as error if empty string, else False -> This will enable move_to_next here: and will continue until a non-empty string is found.

[thekaranacharya]

Tested both with and without streaming for strings, works!