ConfigData with Option.IGNORE_IMPORTS can cause NPE

See https://github.com/spring-cloud/spring-cloud-config/issues/1793#issuecomment-768546224.

We need to change [this line](https://github.com/spring-projects/spring-boot/blob/61502bde9a0c7534cb8e0d16de97a2f2f6d3a00d/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/ConfigDataEnvironmentContributor.java#L227)