Check data availability for Visualization widgets

[longshuicy]

Bug: Empty visualizations show up. For example with the Vega visualization (given a config, show visualization) matches json, but if the config is not set it can't render. It will render the white tile with no content any time there is a JSON file.

We could have a method that the visualization widget/ or developer manually implements to check if all the input data is available and whether the vis widget should be shown or not.