[image_browser] Hides 'Visit Label Feedback' button when no imaging data exist for a session

[shonibare]

To fix #4332

This change hides the 'Visit Label Feedback' button when no imaging data exists for a session

[shonibare]

#4332

I think that is the purpose, to hide the button if no imaging data exists for a candidate. Is EGG data the same as Imaging data?

[jeffersoncasimir]

What I should have said instead is "the button is still visible" when I replace the SessionID in the URL with that of a participant without imaging data, but with EEG data.

EEG data is not the same but both modalities can be associated to the same session ID. It also seems that this line prevents the scenario in the conditional (sessionID being empty string).