Empty pattern field disabled cleanup work <13.6

In older versions of GitLab, the form for adding cleanup policies showed
a `.*` text-box placeholder text for the necessary
field `Remove tags matching` that mislead users into thinking it was
a default value. When submitted, the placeholder did not default to its
shown value and caused a `nil` value to be stored.

A `nil` value for the remove tags pattern effectively
meant no tags would get removed when the policy ran.

This has been fixed in 13.6 (placeholder also acts as default)
and 13.7 (revamp of UI).

doc/user/packages/container_registry/index.md

@@ -666,6 +666,23 @@ For example, you may have two individual images, one for `amd64` and another for
 
 As a workaround, you should include the architecture in the tag name of individual images. For example, use `mygroup/myapp:1.0.0-amd64` instead of using sub repositories, like `mygroup/myapp/amd64:1.0.0`. You can then tag the manifest list with `mygroup/myapp:1.0.0`.
 
+### The cleanup policy doesn't delete any tags
+
+In GitLab 13.6 and earlier, when you run the cleanup policy,
+you may expect it to delete tags and it does not.
+
+This issue occurs when the cleanup policy was saved without
+editing the value in the **Remove tags matching** field.
+
+This field had a grayed out `.*` value as a placeholder.
+Unless `.*` (or other regex pattern) was entered explicitly into the
+field, a `nil` value was submitted. This value prevents the
+saved cleanup policy from matching any tags.
+
+As a workaround, edit the cleanup policy. In the **Remove tags matching**
+field, enter `.*` and save. This value indicates that all tags should
+be removed.
+
 ### Troubleshoot as a GitLab server admin
 
 Troubleshooting the GitLab Container Registry, most of the times, requires