Update tweet_sum_processor.py

[1biswarup2]

Removed two errors from pre-existing code:

1. In the 'get_json()' function[that is in class 'DialogWithSummaries'], the variable 'summ' can be of 'None' then it causes the error.
2. In the 'str()' of the class 'DialogWithSummaries', the 'join' function causes an error when the variable 'abstractive_summary' is not iterable. This exception should be handled and it is done here.

So please merge my changes to the actual code.