Review comments

Author: schmidt-sebastian

src/document.js

@@ -1182,7 +1182,7 @@ class DocumentMask {
   applyTo(data) {
     /*!
      * Applies this DocumentMask to 'data' and computes the list of field paths
-     * that were specified in the mask but not present in 'data'.
+     * that were specified in the mask but are not present in 'data'.
      */
     const applyDocumentMask = data => {
       const remainingPaths = this._sortedPaths.slice(0);
@@ -1210,7 +1210,8 @@ class DocumentMask {
         return result;
       };
 
-      const filteredData = processObject(data);
+      // processObject() returns 'null' if the DocumentMask is empty.
+      const filteredData = processObject(data) || {};
 
       return {
         filteredData: filteredData,

test/document.js

@@ -1038,6 +1038,15 @@ describe('set document', function() {
     );
   });
 
+  it('supports document merges with empty field mask', function() {
+    firestore.api.Firestore._commit = function(request, options, callback) {
+      requestEquals(request, set(document(), updateMask()));
+      callback(null, writeResult(1));
+    };
+
+    return firestore.doc('collectionId/documentId').set({}, {mergeFields: []});
+  });
+
   it('supports document merges with field mask and empty maps', function() {
     firestore.api.Firestore._commit = function(request, options, callback) {
       requestEquals(