empty user comments should be handled on gitcoinbot

[owocki]

see #797 (comment) -- its awkward when the user hasn't entered any text here (or if they used the ioS app)

[arku]

@owocki We've got a validation for the issue action plan but it isn't strict as it doesn't trim the whitespaces. I've submitted a PR which will require a user to input a string for the action plan.

Let me know if it works.

Thanks!