linux: Fix warning when bluez Appearance property isn't set

The Appearance property might not be available, and would cause
warnings like:
upowerd[17733]: g_variant_get_type: assertion 'value != NULL' failed
upowerd[17733]: g_variant_type_is_subtype_of: assertion 'g_variant_type_check (type)' failed
upowerd[17733]: g_variant_get_uint16: assertion 'g_variant_is_of_type (value, G_VARIANT_TYPE_UINT16)' failed
1 job for master in 4 minutes and 7 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #772807
build_stable

00:04:07