Do not throw exception in indexOf() function if PV has not yet connected

[rjwills28]

PR to fix issue described in #3755. This PR doesn't change the underlying behavior but does stop misleading exception messages being printed to the logs.

The initial input variable to the indexOf() function will be given the value: VDouble.of(Double.NaN, Alarm.none(), ...). The function is then evaluated and because the initial value is not of type VEnum, an exception is unnecessarily thrown.

The variable is then given the value VDouble.of(Double.NaN, Alarm.disconnected(), ...) until the PV send its first value. Again, we have not established a connection to the PV yet and so the indexOf() function should not throw an exception.

Under these two cases, value = NaN and alarm = none or disconnected we should not throw an exception if the VType is not VEnum. I have added this logic to the indexOf() function in this PR. Under any other condition we expect this function to be passed a VEnum type and so it would be correct to throw the exception.

I have also added a few tests to provide coverage for this cases.

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