Allow reusing existing "empty" filesystems for /

We currently require that filesystem for / must be newly formatted
by Anaconda, with the new storage workflow the filesystems created
in Cockpit doesn't need to be reformatted so the check needs to
be adjusted to check if the filesystem selected for / is empty and
force the reformatting only when it contains some preexisting data.

pyanaconda/modules/storage/checker/utils.py

@@ -53,7 +53,8 @@ def verify_root(storage, constraints, report_error, report_warning):
                        "which is required for installation of %s"
                        " to continue.") % (get_product_name(),))
 
-    if root and root.format.exists and root.format.mountable and root.format.mountpoint == "/":
+    if root and root.format.exists and root.format.mountable and root.format.mountpoint == "/" \
+       and not root.format.is_empty:
         report_error(_("You must create a new file system on the root device."))
 
     if storage.root_device and constraints[STORAGE_ROOT_DEVICE_TYPES]: