no kernels when envs_dirs contains an empty dir

[Denelvo]

This one took a while to figure out, and it is a border condition.

When envs_dirs in .condarc contains an empty directory, conda is fine. But nb_conda_kernels (through jupyter kernelspec list) always returns an empty list.

This condition happens for example when you're planning on creating a new environment, and you have the dir, but it's not there yet. Because your colleague hasn't provided the spec file yet.

I don't feel confident enough to write the code, but it should be as simple as adding a test in manager.py. I think.