Avoid converting empty strings to NULL pointers in non-Unicode build

...when using utf8_str() or wxGTK_CONV as argument to const char* function parameter

include/wx/string.h

@@ -1709,7 +1709,11 @@ public:
       { return FromUTF8Unchecked(utf8.c_str(), utf8.length()); }
 #endif
     const wxScopedCharBuffer utf8_str() const
-      { return wxMBConvUTF8().cWC2MB(wc_str()); }
+    {
+        if (empty())
+            return wxScopedCharBuffer::CreateNonOwned("", 0);
+        return wxMBConvUTF8().cWC2MB(wc_str());
+    }
 #endif
 
     const wxScopedCharBuffer ToUTF8() const { return utf8_str(); }

src/gtk/utilsgtk.cpp

@@ -86,6 +86,9 @@ wxWindow* wxFindWindowAtPoint(const wxPoint& pt)
 WXDLLIMPEXP_CORE wxCharBuffer
 wxConvertToGTK(const wxString& s, wxFontEncoding enc)
 {
+    if (s.empty())
+        return wxCharBuffer("");
+
     wxWCharBuffer wbuf;
     if ( enc == wxFONTENCODING_SYSTEM || enc == wxFONTENCODING_DEFAULT )
     {
@@ -96,7 +99,7 @@ wxConvertToGTK(const wxString& s, wxFontEncoding enc)
         wbuf = wxCSConv(enc).cMB2WC(s.c_str());
     }
 
-    if ( !wbuf && !s.empty() )
+    if (wbuf.length() == 0)
     {
         // conversion failed, but we still want to show something to the user
         // even if it's going to be wrong it is better than nothing