perf(document): avoid unnecessarily creating new options object on ev…

…ery `$set`

Re: #11541

lib/document.js

@@ -640,7 +640,11 @@ Document.prototype.$__buildDoc = function(obj, fields, skipId, exclude, hasInclu
     for (let i = 0; i < len; ++i) {
       const piece = path[i];
 
-      curPath += (!curPath.length ? '' : '.') + piece;
+      if (!curPath.length) {
+        curPath = piece;
+      } else {
+        curPath += '.' + piece;
+      }
 
       // support excluding intermediary levels
       if (exclude === true) {
@@ -1044,8 +1048,7 @@ Document.prototype.$set = function $set(path, val, type, options) {
     type = undefined;
   }
 
-  options = options || {};
-  const merge = options.merge;
+  const merge = options && options.merge;
   const adhoc = type && type !== true;
   const constructing = type === true;
   let adhocs;
@@ -1055,7 +1058,7 @@ Document.prototype.$set = function $set(path, val, type, options) {
   let key;
   let prefix;
 
-  const strict = 'strict' in options
+  const strict = options && 'strict' in options
     ? options.strict
     : this.$__.strictMode;
 
@@ -1086,7 +1089,7 @@ Document.prototype.$set = function $set(path, val, type, options) {
 
     // `_skipMinimizeTopLevel` is because we may have deleted the top-level
     // nested key to ensure key order.
-    const _skipMinimizeTopLevel = options._skipMinimizeTopLevel || false;
+    const _skipMinimizeTopLevel = options && options._skipMinimizeTopLevel || false;
     if (len === 0 && _skipMinimizeTopLevel) {
       delete options._skipMinimizeTopLevel;
       if (val) {
@@ -1564,7 +1567,7 @@ Document.prototype.set = Document.prototype.$set;
  */
 
 Document.prototype.$__shouldModify = function(pathToMark, path, options, constructing, parts, schema, val, priorVal) {
-  if (options._skipMarkModified) {
+  if (options && options._skipMarkModified) {
     return false;
   }
   if (this.$isNew) {