I have a field which is in date format and when a new item is added to the list the field is blank (no date).
I have a workflow which runs when new items are added. It include the action to "wait for item update" to wait for this field to be manually updated to a date, but the workflow is totally ignoring this action and moves on.
The problem with date fields is that their internal value is never really "empty". Even if you don't place a value in it it will still have some odd date saved that represents an "empty date". So you will have to edit the wait for item update action to be something like "equals today" or "is greater than/equal today" or any other specific value to make it comparable.
What I find more stable than working with dates is to let the workflow update a status text field and wait for a specific status.