feedbackSubmissionEditSave: check recipients for feedback response are valid #2350

Fixes #2350 
Hmmm, I added the "" as a valid recipient since there were existing tests that were checking that editing a response and submitting it with a blank recipient would still be submitted. (such as StudentFeedbackSubmissionEditSaveAction.java line 143 to line 170). Should I change these tests?

The responses would not be saved (the existing tests checks for that as well), since FeedbackQuestionSubmissionEditSaveAction.java 's saveResponse() function doesn't create the feedback response if the recipient is empty. For questions with no specific recipient, it seems like the recipient email "%General%" is used instead.