How do i send an empty FPS or other msg type to HMRC to tell them there were no payments in a pay perod?

Glad to have you here on the Community page, @adennis.

To notify HMRC that there were no payments made in a certain period, you have to run a zero net paycheck and send it to them.

To create a zero net paycheck, follow the steps laid out here: Create a zero net paycheck. This article is for QuickBooks Online (QBO) US version but is also applicable in the UK.

On the other hand, you can also inform them directly by contacting the HMRC Technical Support.

These articles are another great resource about submitting FPS, you may consider reading them:

• Submit FPS to HMRC in QuickBooks Online Standard Payroll (UK) 
• View submitted FPS and EPS filings in QuickBooks Online Standard Payroll 

Let me know if you have any other questions. I'm always here to help!