Event: Simplify the check for saved data in leverageNative

Previously, when `leverageNative` handled async events, there was
a case where an empty placeholder object was set as a result.
Covering both such an object and `false` required a `length` check.
However, this is not necessary since gh-5223 and the check was
already simplified in other places; this one was missed.

Closes gh-5236
Ref gh-5223

(cherry picked from commit dfe212d)

Author: mgol

src/event.js

@@ -570,7 +570,7 @@ function leverageNative( el, type, isSetup ) {
 
 			// If this is a native event triggered above, everything is now in order
 			// Fire an inner synthetic event with the original arguments
-			} else if ( saved.length ) {
+			} else if ( saved ) {
 
 				// ...and capture the result
 				dataPriv.set( this, type, jQuery.event.trigger(