libnm: coerce empty strings to NULL for D-Bus properties
On D-Bus level, string (s) or object paths (o) cannot be NULL. Thus, whenver server exposes such an object, it gets automatically coerced to "" or "/", respectively. On client side, libnm should coerce certain properties back, for which "" is just not a sensible value. For example, an empty NM_DEVICE_ETHERNET_HW_ADDRESS should be instead exposed as NULL. Technically, this is an API change. However, all users were well advised to expect both NULL and "" as possible return values and handle them accordingly.
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