Fix issue with value disappearing when selecting stat

public/app/plugins/panel/gauge/module.tsx

@@ -38,8 +38,8 @@ export const defaultProps = {
     showThresholdLabels: false,
     suffix: '',
     decimals: 0,
-    stat: '',
-    unit: '',
+    stat: 'avg',
+    unit: 'none',
     mappings: [],
     thresholds: [],
   },