Return empty string if task help is blank
Return empty string if task help is blank

Make sure the help text for a task is refreshed when the selected task
changes since a undefined property doesn't cause React to rerender

Closes #1090

fmnhExhibits/Panoptes-Front-End,camallen/Panoptes-Front-End,camallen/Panoptes-Front-End,amyrebecca/Panoptes-Front-End,fmnhExhibits/Panoptes-Front-End,zooniverse/Panoptes-Front-End,CKrawczyk/Panoptes-Front-End,camallen/Panoptes-Front-End,amyrebecca/Panoptes-Front-End,mrniaboc/Panoptes-Front-End,parrish/Panoptes-Front-End,edpaget/Panoptes-Front-End,mrniaboc/Panoptes-Front-End,jelliotartz/Panoptes-Front-End,edpaget/Panoptes-Front-End,alexbfree/Panoptes-Front-End,CKrawczyk/Panoptes-Front-End,edpaget/Panoptes-Front-End,parrish/Panoptes-Front-End,amyrebecca/Panoptes-Front-End,parrish/Panoptes-Front-End,jelliotartz/Panoptes-Front-End,CKrawczyk/Panoptes-Front-End,jelliotartz/Panoptes-Front-End,mrniaboc/Panoptes-Front-End,fmnhExhibits/Panoptes-Front-End,alexbfree/Panoptes-Front-End,alexbfree/Panoptes-Front-End