Call 0 arity function when all arguments to ignoring-nil are nil

[danielcompton]

Previously this would return nil if three or more arguments were provided and all were nil.

The current behaviour looks like a bug, but changing this could potentially be breaking to consumers. I couldn't see this function being used in open source when searching with crossclj. Opening this as a PR to get feedback from others. At the very least this should be noted as a potentially breaking change.

Fixes #38