Make "0" valid number of tags for auto-tagger in trainer.

Pretty straight forward: Currently the minimum number of tags you can make the auto-tagger add is 1. It would be really nice to be able to put “0” so you could use it to set just add tags to every image at the start of the tagging (without calling the autotagger) and more importantly be able to use the prepend and append options to quickly add tags to every picture in training data without autotagger adding something further.