Fix a CRITICAL when calling gimp_layer_mode_get_format() with an
AUTO composite space and a NULL preferred format, which is valid:
it means the layer mode is composite-space agnostic (as DISSOLVE
is), and that there's no preferred format.

A NULL preferred format can occur during
gimp_operation_layer_mode_prepare() if the layer's mode node is not
yet attached anything through its "input" or "aux" pads, which is
the case during the call to gimp_layer_update_mode_node() while
constructing the layer's node in gimp_layer_get_node().