node module: Don't set node-info if version is 'none'

Not sure if this is a recent change, but running `nvm_version` without a node version currently active echoes "none" instead of an empty string; it also returns 0, so we can't check the return value to determine if there is a currently active node version. So, this PR explicitly checks if `'none'` was echoed, and if so, doesn't set the version in `node_info`.

The main benefit of this is to make it easier to not show node-info in themes when no node version is currently in use. In addition, this change makes the node module consistent with the Python module's python-info, which also does not set `python_info` is no virtual environment is currently active.