don't give error when reading empty file in Unicode build

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38466 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/textfile.cpp

@@ -137,8 +137,10 @@ bool wxTextFile::OnRead(wxMBConv& conv)
     }
 
     const wxString str(buf, conv);
+
+    // this doesn't risk to happen in ANSI build
 #if wxUSE_UNICODE
-    if ( str.empty() )
+    if ( bufSize > 4 && str.empty() )
     {
         wxLogError(_("Failed to convert file contents to Unicode."));
         return false;