ATR-22741 - patch for when all triggers in selectChains have the same empty step

remove fully empty steps from all chains.

Should never come up in the full menu, so if the function finds an empty step an exception will be raised if len(self.selectChainsForTesting) == 0 and len(self.signaturesToGenerate).

This will not affect the full menu - it will just make individual testing of tau chains easier.