Workflow shows as Checked out by but with no value. The version shows as Published and cannot be modified.
Checked out a version of the workflow is present in the system which is not published again
In this case, there is a pre-existing checked out version of the workflow which was never published.
As there is a checked-out version the published version is displaying "checked out by" but if we access the checked-out version we will see " "checked out by" followed by the user name.
In order to resolve this, you can either perform a "force checkout" through the contextual menu of the checked-out workflow version and make the changes based on your requirements or publish the workflow if the workflow is as what you are expecting or delete the checked-out version as it only impacts the checked-out user.