bus-message: avoid an infinite loop on empty structures
    
    The alternative would be to treat gvariant and !gvariant messages differently.
    But this is a problem because we check signatures is variuos places before we
    have an actual message, for example in sd_bus_add_object_vtable(). It seems
    better to treat things consistent (i.e. follow the lowest common denominator)
    and disallow empty structures everywhere.
    
    (cherry picked from commit ec6bda56cbca9509b1abde1122645630caca877c)
    
    Resolves: #1696224
    
    patch_name: 0162-bus-message-avoid-an-infinite-loop-on-empty-structur.patch
    present_in_specfile: true
    location_in_specfile: 162
    squash_commits: true