networkd: fia xRequiredOperationalStateForOnline serializtion

networkd: fia xRequiredOperationalStateForOnline serializtion

In integration tests (for example TEST-85-NETWORK-NetworkctlTests)

LINK_OPERSTATE_RANGE_INVALID and required_for_online == -1 are serialized as
```
"RequiredForOnline": "true",
"RequiredOperationalStateForOnline": [null, null]
```
Such link should be reported as required_for_online=False and not
serialize nulls.

(cherry picked from commit 59d946ee6b3612ef2fd646055e39282b0e0189ff)
(cherry picked from commit 2cd1762a1f87096226a66a67e101a6a40295a21c)