Dashboards: Regression with custom variables

### What happened? `VariableModel.query` is used to populate `VariableModel.options`. But `VariableModel.query` does not support defining a named option with an empty value: `unknown : ,trace : 1`. (An empty value in variable is useful, for example, to match entries in Loki without a label.) `unknown : ` is interpreted as a value (instead of name `"unknown"` with value `""`): ```json "options": [ { "selected": false, "text": "unknown :", "value": "unknown :" }, { "selected": false, "text": "trace", "value": "1" } ] ``` `VariableModel.query` is optional according to the schema: https://github.com/grafana/grafana/blob/6ab120a4487a272bc0f3039096a572c7b35f5701/packages/grafana-schema/src/raw/dashboard/x/dashboard_types.gen.ts#L172 So it was possible (in versions < 10.2) to specify `options` directly and leave `query` undefined: ```json "options": [ { "selected": false, "text": "unknown", "value": "" }, { "selected": false, "text": "trace", "value": "1" } ] ``` However, as of version 10.2, this is no longer possible. Since v10.2.4 custom variables without query crash with:  ### What did you expect to happen? To support custom variable with empty value in option again. ### Did this work before? Yes, in versions <= 10.2 (Currently the latest version that supports options with empty values is 10.1.10.) ### How do we reproduce it? 1. Prepare a JSON model of a dashboard with custom variable with either: a) query: `unknown : ,trace : 1` b) or no query and just only options 2. Create a dashboard from a JSON model using the API. 3. When opening the dashboard in Grafana, it will either: a) shows the name of the first option `unknown :` b) fails with the error: `Error updating options: Cannot read properties of undefined (reading 'value')` ### Is the bug inside a dashboard panel? No. ### Environment (with versions)? Graf…