Allow empty objects to be loaded from YAML

[philwebb]

In Spring Boot 4.0.x and earlier OriginTrackedYamlLoader does not add properties for YAML entries that have empty object values.

For example, given the following YAML:

foo:
  bar: test
  baz: {}

A PropertySource is loaded that contains foo.bar but not foo.baz.

With spring-projects/spring-framework#36197 we now have an opportunity to also add foo.baz with an empty String value.

(we should replace null with an empty String to align with .properties and make #48920 easier to implement)

[philwebb]

Blocked until we switch to Framework snapshots