libnm: Don't crash if service tells us a new key management mode

The NetworkManager service sometimes adds new key management modes. If it does, an older client library (perhaps in a container) shouldn't crash when talking to a newer NetworkManager service over D-Bus.

Resolves: #744 (closed)

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This is impossible to test as-is, because there's no newer version of NetworkManager; but it's a forward-port of a tested change made in a package based on libnm 1.6.2, which stopped Steam crashing with this assertion failure when talking to a NetworkManager that supports WPA3-Personal (SAE).