The regression in libnm causes for example nmtui to be unable to edit the cloned-mac-address as NULL.



Reproduce:

Edit an ethernet or wifi connection that has "cloned-mac-address" unset.
Notice, as long as you don't touch the cloned-mac-address input text field in nmtui, you can successfully save the connection (by selecting "OK").
Once you edit the text field, you can no longer clear the value. If you enter an empty text (which should be a supported value), the server will reject saving the connection.




to quote the commit-message from the patch:

>>>

    libnm: fix regression serializing empty "cloned-mac-address"
    
    For "cloned-mac-address", the empty string "" is an invalid
    value that is rejected by verify().
    
    Commit 8eed671 changed how the property is serialized to D-Bus.
    Before, it was serialized using _nm_utils_hwaddr_to_dbus().
    For invalid or empty addresses, this would not serialize the
    value on D-Bus (or before commit 76aa6f8e0, it would create
    a bogus value with no array elements).
    
    With commit 8eed671, the cloned-mac-address gets also serialized
    as "assigned-mac-address" via _nm_utils_hwaddr_cloned_data_synth(),
    which would pass on invalid strings that the server would then reject.
    
    That breaks for example nmtui. Try editing a connection with
    "cloned-mac-address" set to NULL. Note, as long as you don't edit
    the cloned MAC address in nmtui, you can save the modification.
    Once you start modifying the entry, you can no longer set an empty
    MAC address as the server now receiveds the empty string as
    invalid. Thus, the "OK" button fails with
       Unable to save connection:
       802-3-ethernet.cloned-mac-address:
       is not a valid MAC address
    It also means, nmtui cannot modify the "cloned-mac-address" field to
    become empty.
    
    Fix that problem at various places by coercing "" to NULL.