Fix #935: Machine output slots getting stuck in an invisible locked state

[Technici4n]

Issue #935 could be reproduced with the following procedure:

• Set capacity of the first 3 slots to 0.
• Insert a single iron ingot and wait for the dust to appear in the 4th slot.
• Increase capacity of first slots back to 64.
• Insert a single copper ingot.
• Notice that no recipe starts.

The problem comes from stack.setKey(output.variant()); being called while ins is 0. The fix is to add a check for ins > 0.