Handle nil cleanup functions (#1512)

Because I don't have an exact version, I can't be 100% sure, but I
believe the stack trace in the ticket ENG-320 Is caused by this cleanup
function being nil. Either way, it seems like a good thing to do to
doublecheck before deferring a function that it is actually set

Fixes ENG-320

GitOrigin-RevId: a2e1057bd6d54f2238fbb85cb814255eea4a0fd8

Author: Dylan

cmd/terraform_plan.go

@@ -74,7 +74,9 @@ func TerraformPlan(cmd *cobra.Command, args []string) error {
 	if err != nil {
 		return err
 	}
-	defer cleanup()
+	if cleanup != nil {
+		defer cleanup()
+	}
 
 	return TerraformPlanImpl(ctx, cmd, oi, args, planFile)
 }