ENH: Handle the no-icon case

Modules/NSIS.template.in

@@ -394,7 +394,10 @@ FunctionEnd
 ;--------------------------------
 ; Define some macro setting for the gui
 
-  !define MUI_HEADERIMAGE_BITMAP "@CPACK_PACKAGE_ICON@"
+  !define CPACK_PACKAGE_ICON@CPACK_PACKAGE_ICON@
+  !ifndef CPACK_PACKAGE_ICON
+    !define MUI_HEADERIMAGE_BITMAP "@CPACK_PACKAGE_ICON@"
+  !endif
 
 ;--------------------------------
 ;Pages