Fix reg cannot scan vulnerabilities on images with empty layers

[KeyboardNerd]

The name of layer was not properly computed because some empty layer causes the layer map to have missing indexes.

Fixes #167

[jessfraz]

@jzelinskie can you make sure this seems right to you too, thanks!

[jzelinskie]

I think this change is correct, but it took me far longer to understand than it should. The filtering stuff needs to get pulled out into its own function and maybe be applied just before POSTing. That way we can keep track of these invariants in one place, rather than all over the place.

[KeyboardNerd]

I don't understand why it's hard to understand but yes, the empty layer filtering can be extracted from the function to make it more clear. I'll update the PR.

[KeyboardNerd]

cool fixed @jzelinskie