Stop using "NULL" in the minimal sample

This should have been part of 4f4c5fcfdf (Use nullptr instead of NULL in
the code and documentation, 2022-10-16).

samples/minimal/minimal.cpp

@@ -139,7 +139,7 @@ bool MyApp::OnInit()
 
 // frame constructor
 MyFrame::MyFrame(const wxString& title)
-       : wxFrame(NULL, wxID_ANY, title)
+       : wxFrame(nullptr, wxID_ANY, title)
 {
     // set the frame icon
     SetIcon(wxICON(sample));