Normalize constrain in "PriorityHelper" class, only when it's specified

In 5.2.0-B2 version we've introduced PriortyEventHandler class, that allows to sort items in list like it's done for categories.

Key differences from what previous implementations:

• buttons MoveUp/MoveDown in toolbar really move selected list items one position up and down and not just change Priority column value to +1 or -1 resulting items with duplicate priorities
• no need to guess what priority to set, since now it's a dropdown from all available priorities and you can easily select that a specific item must at Nth position in list
• easy to use dropdown filter in list (instead of just a textbox)

In default installation there is no place, where this is implemented except for Categories and Promo Blocks. However there is not "Recalculate Priorities" toolbar button in promo blocks. If there was a such button, then clicking on it would produce an SQL error.