Create empty wxCFStringRef and not null one if conversion fails

If the passed string cannot be represented in the target encoding in the
wxCFStringRef constructor, create a reference to an empty string instead of a
null ref. Most users of wxCFStringRef cannot handle a null wxCFStringRef.

Closes #17825.

(cherry picked from commit a2b04536d3)

src/osx/core/cfstring.cpp

@@ -621,9 +621,16 @@ wxCFStringRef::wxCFStringRef( const wxString &st , wxFontEncoding WXUNUSED_IN_UN
 #else
     #error "unsupported Unicode representation"
 #endif
-
-        reset( CFStringCreateWithBytes( kCFAllocatorDefault,
-            (const UInt8*)data, size, cfencoding, false /* no BOM */ ) );
+        CFStringref ref = CFStringCreateWithBytes( kCFAllocatorDefault,
+            (const UInt8*)data, size, cfencoding, false /* no BOM */ );
+        if (ref)
+        {
+            reset( ref );
+        }
+        else
+        {
+            reset( wxCFRetain( CFSTR("") ) );
+        }
 #else // not wxUSE_UNICODE
         reset( CFStringCreateWithCString( kCFAllocatorSystemDefault , str.c_str() ,
             wxMacGetSystemEncFromFontEnc( encoding ) ) );