NULL == X is just another coding style used by the programmers to avoid some mistakes in codes.
Whenever people write bigger codes they use to follow these type of convention instead of X == NULL reason being while writing code the programmer may type X = NULL instead of X == NULL which will result in overwriting of X.
While in other case NULL ==X if accidently programmer writes NULL = X then he/she will get a compilation error instead overwriting.
you have to generate features from the tweets.
eg: some hashtag says #serious so you can use this type of info in performing your sentiment analysis.