Avoid crash when no data is returned from query

[ghost]

In the case where no data is returned, the plugin crashes. Then, when the time range is changed and data is again available, it does not automatically recover. A full page reload is required to clear the error.

Expected behavior: when there's no data returned from the query to compute the word cloud, don't attempt to render; just show "No Data" or similar

Actual behavior: JS crash with red !, requiring full page reload to recover

Error message: TypeError: Cannot read property 'fields' of undefined
at eval (/public/plugins/magnesium-wordcloud-panel/module.js:2)
at na (vendorsapp.0cd8deb042c68e8113e4.js:2)
at La (vendorsapp.0cd8deb042c68e8113e4.js:2)
at Uu (vendorsapp.0cd8deb042c68e8113e4.js:2)
at ks (vendorsapp.0cd8deb042c68e8113e4.js:2)
at ws (vendorsapp.0cd8deb042c68e8113e4.js:2)
at _s (vendorsapp.0cd8deb042c68e8113e4.js:2)
at ds (vendorsapp.0cd8deb042c68e8113e4.js:2)
at vendorsapp.0cd8deb042c68e8113e4.js:2
at t.unstable_runWithPriority (vendors~app.0cd8deb042c68e8113e4.js:2)
ou @ react-dom.production.min.js:216
getDiagramSeriesModel.tsx:134

Browser: Chrome 92
OS: MacOS 11.3.1