Allow "incomplete" env_variable_switches to be merged

[JohnBernas]

Before, if some switch was missing either recipes, cookbook_paths or node_attributes, it would error out when merging, with one of the following errors:

• no implicit conversion of nil into Array
• undefined method `each_pair' for nil:NilClass

depending on the one missing.

[JohnBernas]

Travis CI broke the build for some reason, I doubt is related to these changes though...