Statistics: Don't blow up when annotating/highlighting stuff with the plugin loaded, but disabled (#6829)

Re #6825
Re #6827

plugins/statistics.koplugin/main.lua

@@ -2221,20 +2221,24 @@ function ReaderStatistics:onCloseDocument()
 end
 
 function ReaderStatistics:onAddHighlight()
+    if self.is_enabled then
         self.data.highlights = self.data.highlights + 1
-    return true
+    end
 end
 
 function ReaderStatistics:onDelHighlight()
+    if self.is_enabled then
         if self.data.highlights > 0 then
             self.data.highlights = self.data.highlights - 1
         end
-    return true
+    end
 end
 
 function ReaderStatistics:onAddNote()
+    if self.is_enabled then
         self.data.notes = self.data.notes + 1
     end
+end
 
 -- Triggered by auto_save_settings_interval_minutes
 function ReaderStatistics:onSaveSettings()