[Regression] Ensure that Chef::Config[:knife][:hints]['linode'] exists.

In a version of Chef between 11.6.0 and 11.8.2 the options default
behavior changed such that the linode hints hash was not initialized,
causing an "undefined method `merge!' for nil:NilClass'`" error to be
raised when creating a server with Chef 11.8.2.

This fix guards against the linode hash being nil just before the
in-place merge operation occurs. Is backwards compatible with prior
versions of Chef that worked with the previous option logic.