The fields configured in /modules/workflow/dialogs/publish/form/tabs/comment/fields don't accept validators, so no standard validation (i.e. required=true for comment) is possible at the moment.

From a support ticket:

Zdenek Skodik added a comment - 04/Jul/14 12:19 PM

Hi,

the workflow dialogs are kind of specific on purpose dialogs, so that they have their own presenter class that is partially responsible for the fact that the common dialog validation doesn't work there - see /modules/workflow/dialogs/publish/presenterClass. It binds the involved elements - activation actions, forms, passing parameters (comment, scheduled date) around, creating the form actions ad hoc. So at the moment the validation can only be provided by code, e.g. by overriding the #executeAction() of the presenter. Feel free to open an improvement ticket at the workflow project.

Kind regards,
Zdenek