Gracefully handle empty configuration files

[rjocoleman]

Currently if ~/.gemstash/config.yml (or ~/.gemstash/config.yml.erb) exists but has no yaml content the YAML.load_file method returns nil.

Failure/Error: @config = DEFAULTS.merge(@config)

TypeError:
  no implicit conversion of nil into Hash
# ./lib/gemstash/configuration.rb:36:in `merge'

This PR sets a default value of an empty hash, this is used in the case that the parsed config file is empty.
I also introduced a next test case for this.

I encountered this when using a dynamically generated config.yml that had no content (so defaults were expected).

I also made a small change to the method, removing the variable assignment as the values are returned. and assigned to that variable in the callers.