[NPW] Project config marked as secret produces encrypted empty string in stack config if no value is provided instead of not setting a value.

[MitchellGerdisch]

Expected behavior

When going through the NPW flow, if there is a template config for a secret stack config that is not set by the user the stack config file is configured with an encrypted empty string. The expected behavior would be that the stack config file should not contain the value at all. This way the pulumi program can note this as being unset and react accordingly.

Current behavior

Stack config gets an encrypted empty string.

Steps to reproduce

1. Create a small project template with a secret value. I would expect this to work: https://github.com/pulumi/examples/tree/master/azure-py-webserver
2. Run NPW in the UI using that template but don't set the password.
3. Look at the generated stack config and note password has a value (which is blank).

Context (Environment)

Affected feature