[PATCH 1/2] lib-storage: Allow NULL path in mailbox_list_mkdir_root()

[PATCH 1/2] lib-storage: Allow NULL path in mailbox_list_mkdir_root()

When path is NULL, look up the root directory path of the given type via
mailbox_list_get_root_path() and create that. If there is no path of the
requested type, nothing is done. This avoids open-coding the common
get_root_path() + mkdir_root() pair at call sites.

Gbp-Pq: Name 0001-lib-storage-Allow-NULL-path-in-mailbox_list_mkdir_ro.patch