category creation needs nil protection

[zeptognome]

Addon Version

v0.1.32

World of Warcraft Version

Retail

Describe the bug

In conjunction with #352, when dragging a locked item to the bag icon to make a custom category, this lua error is generated

1x BetterBags/frames/bag.lua:308: bad argument #2 to 'format' (string expected, got nil)
[string "=[C]"]: in function `format'
[string "@BetterBags/frames/bag.lua"]:308: in function <BetterBags/frames/bag.lua:305>

Locals:
(*temporary) = "What would you like to name the new category for %s?"
(*temporary) = nil
(*temporary) = "string expected, got nil"

Steps to reproduce

cause an item to be locked via item interaction (placing an item in the mail, AH, or catalyst UIs)
drag the item to the bag icon to create a category

[zeptognome]

no longer an issue as betterbags don't touch itemlocks