not using dir as base dir if it's empty

[jeffastorey]

Proposed check to see if the path is empty and use the working dir if it is. This came out of a discussion I had with @bradrydzewski around how to call terraform in the build step of a drone build. The suggested way (which should be easier in 0.5) was to use the drone-terraform image and create the plugin config in json, something like

/bin/drone-terraform -- '{"vargs":{"plan":true...}'

When calling this way, path is empty.

[jeffastorey]

@jackspirou could you take a look at this?

[tboerger]

LGTM

[nlf]

LGTM