block unnecessary calls to api when production editor is unassigned

app/components/BookBuilder/BookBuilder.jsx

@@ -72,6 +72,9 @@ export class BookBuilder extends React.Component {
     let patch
 
     if (_.isEmpty(productionEditors)) {
+      // production editor is already set to null
+      if (book.productionEditor === null) return
+
       patch = {
         id: book.id,
         productionEditor: null