Add nullStep as return for multistep.If (#10247)

This change introduces a new nullStep type that can be used in place of
a nil step. This fixes an issue where multistep.If would return a nil
step if the condition was not met causing a builder to crash when
executed using `-on-error=ask` or `-on-error=abort`.

Closes #10241

helper/multistep/if.go

@@ -3,7 +3,7 @@ package multistep
 // if returns step only if on is true.
 func If(on bool, step Step) Step {
 	if on == false {
-		return nil
+		return &nullStep{}
 	}
 	return step
 }

helper/multistep/multistep.go

@@ -61,3 +61,11 @@ type Runner interface {
 	// Run runs the steps with the given initial state.
 	Run(context.Context, StateBag)
 }
+
+type nullStep struct{}
+
+func (s nullStep) Run(ctx context.Context, state StateBag) StepAction {
+	return ActionContinue
+}
+
+func (s nullStep) Cleanup(state StateBag) {}