fix: nothing selected bug (#6260)

* fix: nothing selected bug

* fix: consolidate related logic for audience settings

* simplify

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Co-authored-by: Dan Abramov <[e-mail]>

src/components/dialogs/PostInteractionSettingsDialog.tsx

···
     } else {
       newSelected.splice(i, 1)
     }
+    if (newSelected.length === 0) {
+      newSelected.push({type: 'everybody'})
+    }
 
     onChangeThreadgateAllowUISettings(newSelected)
   }