Don't use DDEExec registry key in wxMSW wxExecute() if it's empty.

Some file types have DDEExec subkey in the registry but no value for it, don't
use DDE for launching the files of these types in this case as this only
results in errors.

Closes #15388.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74636 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -574,6 +574,7 @@ All (GUI):
 wxMSW:
 
 - It is now possible to tab into radio boxes again.
+- Fix launching some types of files under Windows 7 and later (Steven Houchins).
 
 wxOSX:
 

src/msw/mimetype.cpp

@@ -291,6 +291,10 @@ wxString wxFileTypeImpl::GetCommand(const wxString& verb) const
             if ( keyDDE.Open(wxRegKey::Read) ) {
                 wxString ddeCommand, ddeServer, ddeTopic;
                 keyDDE.QueryValue(wxEmptyString, ddeCommand);
+
+                // in some cases "DDEExec" subkey exists but has no value, we
+                // shouldn't use DDE in this case
+                if ( !ddeCommand.empty() ) {
                     ddeCommand.Replace(wxT("%1"), wxT("%s"));
 
                     wxRegKey keyServer(wxRegKey::HKCR, strKey + wxT("\\Application"));
@@ -309,6 +313,7 @@ wxString wxFileTypeImpl::GetCommand(const wxString& verb) const
                             << wxT('#') << ddeTopic
                             << wxT('#') << ddeCommand;
                 }
+            }
             else
 #endif // wxUSE_IPC
             if ( !foundFilename )