create-key with no passphrase with pop-up loop

[James Jesudason]

Running snapcraft create-key without a passphrase (as is needed for the serial-vault) is not possible on a VM.

1. Create a Ubuntu 18.04 EC2 instance.
2. ssh to the instance and try running `snapcraft create-key`
3. Press enter as the 2 passphrase prompts
4. A pop-up is shown asking for a passphrase
5. Tab and enter on the OK button redisplays the pop-up

Is there a way around this? The customer uses MacOS to connect to the EC2 instance to generate the keys.