udev/net_id: add debug logging for construction of device names

I think this makes it much easier to figure out what information sources
were used to generate the names, and why certain names were not generated.

On my laptop:

Using default interface naming scheme 'v250'.
ID_NET_NAMING_SCHEME=v250
wwp0s20f0u2i12: addr_assign_type=3, MAC address is not permanent.
wwp0s20f0u2i12: Parsing slot information from sysname "0000:00:14.0": success
wwp0s20f0u2i12: dev_port=0
wwp0s20f0u2i12: PCI path identifier: domain=0 bus=0 slot=20 func=0 phys_port= dev_port=0 → p0s20f0
wwp0s20f0u2i12: USB name identifier: ports=2 config=1 interface=12 → u2i12
ID_NET_NAME_PATH=wwp0s20f0u2i12

Using default interface naming scheme 'v250'.
ID_NET_NAMING_SCHEME=v250
ID_NET_NAME_MAC=en54ee75cb1dc0
enp0s31f6: MAC address identifier: hw_addr=54:ee:75:cb:1d:c0 → 54ee75cb1dc0
ID_OUI_FROM_DATABASE=Wistron InfoComm(Kunshan)Co.,Ltd.
enp0s31f6: Parsing slot information from sysname "0000:00:1f.6": success
enp0s31f6: dev_port=0
enp0s31f6: PCI path identifier: domain=0 bus=0 slot=31 func=6 phys_port= dev_port=0 → p0s31f6
ID_NET_NAME_PATH=enp0s31f6

Using default interface naming scheme 'v250'.
ID_NET_NAMING_SCHEME=v250
ID_NET_NAME_MAC=en0050b6856d36
hub0: MAC address identifier: hw_addr=00:50:b6:85:6d:36 → 0050b6856d36
ID_OUI_FROM_DATABASE=GOOD WAY IND. CO., LTD.
hub0: Parsing slot information from sysname "0000:00:14.0": success
hub0: dev_port=0
hub0: PCI path identifier: domain=0 bus=0 slot=20 func=0 phys_port= dev_port=0 → p0s20f0
hub0: USB name identifier: ports=4.1.3 config=2 interface=0 → u4u1u3c2
ID_NET_NAME_PATH=enp0s20f0u4u1u3c2

Using default interface naming scheme 'v250'.
ID_NET_NAMING_SCHEME=v250
wlp4s0: addr_assign_type=3, MAC address is not permanent.
wlp4s0: Parsing slot information from sysname "0000:04:00.0": success
wlp4s0: dev_port=0
wlp4s0: PCI path identifier: domain=0 bus=4 slot=0 func=0 phys_port= dev_port=0 → p4s0
ID_NET_NAME_PATH=wlp4s0

src/basic/string-util.h

@@ -60,6 +60,10 @@ static inline const char *empty_to_null(const char *p) {
         return isempty(p) ? NULL : p;
 }
 
+static inline const char *empty_to_na(const char *p) {
+        return isempty(p) ? "n/a" : p;
+}
+
 static inline const char *empty_to_dash(const char *str) {
         return isempty(str) ? "-" : str;
 }