Fix inconsistent plan error

This is a temprorary(?)/partial(?) fix for the inconsistent plan error.

As part of creating the examples for (#203) I was running into this
issue and noticed that all of the occurrences were due to the
`terraform_data` resources changing from `null` to `""`. Adding this
simple `try` function to always fallback to `""` seems to have fixed the
issue in a lot of cases.

This obviously doesn't fix the inconsistent plan issue in all cases, but
it gets us unstuck for creating example tests.

re #198

Author: corymhall

pkg/tfsandbox/tf_file.go

@@ -214,7 +214,7 @@ func CreateTFFile(
 
 		resourceName := fmt.Sprintf("%s%s", terraformDataResourcePrefix, output.Name)
 		resources[terraformDataResourceType][resourceName] = map[string]interface{}{
-			"input": fmt.Sprintf("${module.%s.%s}", name, output.Name),
+			"input": fmt.Sprintf("${try(module.%s.%s, \"\")}", name, output.Name),
 		}
 	}