Previously, the default value would lead to a wrong-type-to-apply crash.
* gnu/system/image.scm (system-disk-image)[image-builder]: When
'partition-initializer' returns #f, fall back to
INITIALIZE-ROOT-PARTITION.
* gnu/tests/base.scm (run-root-unmount-test)[test-image]: Remove
'initializer' field of partition.
* gnu/image.scm (<partition>)[initializer]: Add comment.