fix(ngStyle): skip setting empty value when new style has the property

Previously, all the properties in oldStyles are set to empty value once.
Using AngularJS with jQuery 3.3.1, this disables the CSS transition as
reported in jquery/jquery#4185.

Author: itchyny

src/ng/directive/ngStyle.js

@@ -54,7 +54,11 @@
 var ngStyleDirective = ngDirective(function(scope, element, attr) {
   scope.$watchCollection(attr.ngStyle, function ngStyleWatchAction(newStyles, oldStyles) {
     if (oldStyles && (newStyles !== oldStyles)) {
-      forEach(oldStyles, function(val, style) { element.css(style, '');});
+      forEach(oldStyles, function(val, style) {
+          if (!(newStyles && newStyles.hasOwnProperty(style))) {
+              element.css(style, '');
+          }
+      });
     }
     if (newStyles) element.css(newStyles);
   });