Better handling of latest parameter values when no data is available

[akhenry]

Closes nasa/openmct#5385

Describe your changes:

This bug was due to a recent unrelated change in our parameter subscriptions which means that Yamcs now sends us back the latest value for a parameter immediately. If Yamcs has nothing in its latest data table (because no parameter value has been received since the last restart) then Yamcs will return an slightly different response, which was being incorrectly interpreted as a telemetry value.

Note that fixing this issue unearthed another problem, which I have also fixed in a separate PR.

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[shefalijoshi]

Looks good