/application/env/{propertyName} returns 200 for a property that does not exist

[snicoll]

If you query the env endpoint for a property and that property doesn't exist (Environment#containsProperty returns false), the web endpoint returns a 200.

We could return null in the endpoint method to make it return a 404 instead but we'd lose the information about which property sources are active.

[wilkinsona]

See also #10178