INTEG-238 [integ-forum][migration] Broken Activity Stream due to NumberFormatException in AnswerUIActivity

Fix description:
* Handle empty string when converting string to a number in the methods: getRating, getNumberOfAnswers, getNumberOfComments
INTEG-238 [integ-forum][migration] Broken Activity Stream due to NumberFormatException in AnswerUIActivity
INTEG-238 | Fix link of question created in space on activity
INTEG-238 [integ-forum][migration] Nested activities due to NumberFormatException inside AnswerUIActivity.gtmpl

Fix description:
- Handle empty string when converting string to a number in the methods: getRating, getNumberOfAnswers, getNumberOfComments.
- On migration, if the activity is not exist or the current activity is not the same as the activity associated to the question, then just add comment on this activity.