Dashboard widget deserialization: gmap widget with null 'scale' throws DeserializationException

[davidnmbond]

Summary

Deserializing a dashboard that contains a Google Map (gmap) widget whose scale is null throws:

DeserializationException: Error converting value {null} to type 'System.Single'. Path 'scale'\n```\n\nThis fails the whole dashboard/widget fetch, so listing a dashboard's widgets is impossible if any gmap widget has a null scale.\n\n## Cause / fix\nThe gmap widget model maps `scale` as a non-nullable `float`/`System.Single`. Google-map widgets can legitimately return `scale = null`. Change the property to nullable (`float?`).\n\n## Impact\nAny dashboard containing a gmap widget with a null scale cannot be deserialized; callers must fall back to raw REST. Reproduced against a live portal via the graph/dashboard widget read path.