Split "No value" in time series panel into "no metrics at all" and "default value when the metric is not present"

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: So, I am using "No value" in my panels to avoid showing metrics that have 0 value during the displayed timespan. Usually I write Prometheus queries like `rate(timeout_events_total[1m]) > 0`, so it won't display metrics that did not change during the specified timespan, and I use "No value" as 0 so it would assume 0 as the default value where the metric is not present. This works exactly how I want it, for example, here's how one of my panels looks like without this field:  and with this field:  However, if there are 0 time series, it would display the same value, and it would look like this:  This seems kinda ugly and I'd rather want to be able to specify here something like "No metrics", but if I put "No metrics" in the "No value" field, it would display "No metrics" if there are zero time series on a panel, but would leave gaps in the panel if there are time series that have no values at some point. **What would you like to be added**: Split the "No value" feature into 2 fields: 1) a value used when there are 1 or more time series that are missing data at some points as a fallback value (and I assume it should be numeric only) 2) a value used as a placeholder to draw instead of the panel itself if there are 0 time series. **Who is this feature for?**