[PERF] Don't Attempt To Sanitive Unsanitizeable

This adds an identity check for falsy values before the value is
sanitized. Currently we do work to sanitize things like `undefined`.

Author: chadhietala

packages/morph-attr/lib/main.js

@@ -110,7 +110,7 @@ AttrMorph.prototype.setContent = function (value) {
   if (this.lastValue === value) { return; }
   this.lastValue = value;
 
-  if (this.escaped) {
+  if (this.escaped && this.valueIdentity(value)) {
     var sanitized = sanitizeAttributeValue(this.domHelper, this.element, this.attrName, value);
     this._update(sanitized, this.namespace);
   } else {
@@ -123,6 +123,11 @@ AttrMorph.prototype.getContent = function () {
   return value;
 };
 
+// Used in conditional expression for truthiness
+AttrMorph.prototype.valueIdentity = function (value) {
+  return value;
+};
+
 // renderAndCleanup calls `clear` on all items in the morph map
 // just before calling `destroy` on the morph.
 //