Ensure doc is defined before accessing properties

Author: phillmac

src/DocumentIndex.js

@@ -15,7 +15,7 @@ class DocumentIndex {
     const reducer = (handled, item, idx) => {
       if (item.payload.op === 'PUTALL' && item.payload.docs[Symbol.iterator]) {
         for (const doc of item.payload.docs) {
-          if (handled[doc.key] !== true) {
+          if (doc && handled[doc.key] !== true) {
             handled[doc.key] = true
             this._index[doc.key] = {
               payload: {