Remove isNonEmptyPlan from the Plan() method of the Executor interface

[ZIJ]

We no longer use it; in #1028 we switched to parsing actual plan output json to determine whether or not it's empty
So the is confusing

type Executor interface {
	Plan() (bool, bool, string, string, error)
	Apply() (bool, string, error)
	Destroy() (bool, error)
}

[Jyoti050402]

Hey @ZIJ I want to work on this issue. Can you please assign me?

[ty-labs]

Should this issue be removed? Seems like it is stale as isNonEmptyPlan looks like it is used now:

digger/cli/pkg/digger/digger.go

Lines 268 to 281 in 1d592f9

	planSummary, planPerformed, isNonEmptyPlan, plan, planJsonOutput, err := diggerExecutor.Plan()
	if err != nil {
	msg := fmt.Sprintf("Failed to Run digger plan command. %v", err)
	log.Printf(msg)
	err := prService.SetStatus(*job.PullRequestNumber, "failure", job.ProjectName+"/plan")
	if err != nil {
	msg := fmt.Sprintf("Failed to set PR status. %v", err)
	return nil, msg, fmt.Errorf(msg)
	}
	return nil, msg, fmt.Errorf(msg)
	} else if planPerformed {
	if isNonEmptyPlan {