Fix handling of "Cancel" button in wxSearchCtrl under OS X.

Pressing it results in a control action with an empty, but not null, string.
Recognize it correctly.

See #16869.

src/osx/cocoa/srchctrl.mm

@@ -155,7 +155,7 @@ public :
         if ( wxpeer )
         {
             NSString *searchString = [m_searchField stringValue];
-            if ( searchString == nil )
+            if ( searchString == nil || !searchString.length )
             {
                 wxpeer->HandleSearchFieldCancelHit();
             }