Update `ConfigDataEnvironmentContributor.isActive` so that unbound
imports are no longer considered active. Prior to this commit, any
`ConfigDataEnvironmentContributor` that had `null` properties was
considered active. This is incorrect for `Kind.UNBOUND_IMPORT`
contributors since we haven't yet bound the `spring.config.*`
properties.
The `ConfigDataEnvironmentContributorPlaceholdersResolver` has been
updated to handle the refined logic. A placeholder can now be resolved
from the current contributor, or from an unbound contributor by binding
it on the fly.
Fixes gh-29386
3d46b06e8d