Fixed NPE when one type of token is not present on TokenizedCharSequence 

When using TokenizedCharSequence, if a parsed tweet doesn't contains a specific type of token getTokensOf(TokenType... types) will return null. When called from getTokenStringsOf(TokenType... types) or within the for (TokenType type : types) loop it will result in a NullPointerException.

This change returns an empty list instead of null, which prevents the exception.