Assets "saved with empty engine version" warning

Thankyou SunMachineYanny. I ran the command you suggested and it worked very nicely so all my warnings are now gone. I would just add that the project must not be open for the command to work. It will run but have no effect if the project is open.
Where did you find out about this may I ask? I often feel there is a secret store of information that I am not privy to.