Form switching fails when requested_slot is the same in both forms

[desmarchris]

https://rasa-hq.slack.com/archives/C7FCS16SD/p1608240711266000

Rasa version: 2.1.2

Rasa SDK version (if used & relevant): 2.1.2

Python version: 3.8.6

Operating system (windows, osx, ...): osx

Issue:
When switching between two forms, if the requested_slot is the same in the new and old form, the bot returns an empty response rather than switching to the new form

The issue can be reproduced by pulling this commit from the retail-demo, and then try:

Your input ->  order status
I can help you find the status of your order. What's your email address so I can find the order?
Your input ->  return
Your input ->

The expected conversation must be:

Your input ->  order status
I can help you find the status of your order. What's your email address so I can find the order?
Your input ->  return
Let's start that return. To help find your order, can you please provide your email address?
Your input ->

[TyDunn]

@ArjaanBuijk I am closing this because your PR said it fixed this