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.

Closes #16869.

(this is a backport of 7065e26fb3 from master)

docs/changes.txt

@@ -637,6 +637,7 @@ wxOSX:
 - Fix using wxHTTP and wxFTP from worker thread.
 - Fix wxFileDialog::GetFilterIndex() for file open dialogs (phsilva).
 - Fix wxSearchCtrl appearance under 10.10 (John Roberts).
+- Fix handling of "Cancel" button in wxSearchCtrl (John Roberts).
 - Generate correct events for WXK_NUMPAD_ENTER (John Roberts).
 - Fix handling of WXK_NUMPAD_ENTER in wxTextCtrl (John Roberts).
 - Send wxIconizeEvent when a window is iconized/restore (Rob Krakora).

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();
             }