Remove check for invalid return, update spec

Remove a check for an invalid return value, update the spec to
return the proper value.  Also updated the subscribe method
to fetch defaults the same as the request method.

https://github.com/nasa/openmct/pull/1544

platform/telemetry/test/TelemetryCapabilitySpec.js

@@ -92,7 +92,7 @@ define(
                 ]);
                 mockTelemetryAPI.getMetadata.andReturn({
                     valuesForHints: function () {
-                        return [];
+                        return [{}];
                     }
                 });