As of Spring Boot 2.0, if Collection properties are specified in
multiple property sources, only the elements from the property source
with the highest precedence are used for binding. This caused an
`UnboundConfigurationPropertiesException` if the size of the collection
from the higher order property source was smaller and `ignoreUnknownFields`
was set to true.
This commit ignores unbound collection properties if the
collection was properly bound.
Fixes gh-16290
91c1fc3d97