Eliminate redundancy of Deferred.then().

src/deferred.js

@@ -21,24 +21,19 @@ jQuery.extend({
 					return jQuery.Deferred(function( newDefer ) {
 						jQuery.each( tuples, function( i, tuple ) {
 							var action = tuple[ 0 ],
-								fn = fns[ i ];
+								fn = jQuery.isFunction( fns[ i ] ) && fns[ i ];
 							// deferred[ done | fail | progress ] for forwarding actions to newDefer
-							deferred[ tuple[1] ]( jQuery.isFunction( fn ) ?
-								function() {
-									var returned = fn.apply( this, arguments );
+							deferred[ tuple[1] ](function() {
+								var returned = fn && fn.apply( this, arguments );
 								if ( returned && jQuery.isFunction( returned.promise ) ) {
 									returned.promise()
 										.done( newDefer.resolve )
 										.fail( newDefer.reject )
 										.progress( newDefer.notify );
 								} else {
-										newDefer[ action + "With" ]( this === promise ? newDefer.promise() : this, [ returned ] );
+									newDefer[ action + "With" ]( this === promise ? newDefer.promise() : this, fn ? [ returned ] : arguments );
 								}
-								} :
-								function() {
-									newDefer[ action + "With" ]( this === promise ? newDefer.promise() : this, arguments );
-								}
-							);
+							});
 						});
 						fns = null;
 					}).promise();