Empty levels should breaks latches

[wismill]

Testing the German E1 with its modifiers changed as Latches, I found that empty levels do not break latches. Indeed, we skip our for loops in filters because the count is zero! 😅 This is a regression introduced in #487.

Why break latches with empty levels? In essence they are levels with just a NoSymbol keysym, and NoAction breaks latches.

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Discovered indirectly thanks to @Jules-Bertholet who mentioned the E1 layout.