mm_firmware_update_settings_get_variant() checks for a null `self'
pointer when accessing `self->priv->method', but doesn't perform the
null check when accessing other members of
MMFirmwareUpdateSettingsPrivate.
This patch fixes mm_firmware_update_settings_get_variant() to fully
handle a null `self' pointer.
No need to process the detailed dictionary if no explicit method is
reported as supported. Avoids unnecessary warnings:
$ mmcli -m 1 --firmware-status
** (mmcli:6887): WARNING **: 15:52:54.664: Invalid initial update settings: Missing required 'device-ids' setting
error: firmware status unsupported
