Use wxString() instead of "" for empty strings

This will allow this code to work even when implicit conversion from
"const char*" is disabled in wxString and is already marginally more
efficient even now.

See https://github.com/wxWidgets/wxWidgets/pull/782

include/wx/bmpbuttn.h

@@ -59,7 +59,7 @@ public:
         // And we also use wxBU_EXACTFIT to avoid being resized up to the
         // standard button size as this doesn't make sense for bitmap buttons
         // which are not standard anyhow and should fit their bitmap size.
-        return wxButton::Create(parent, winid, "",
+        return wxButton::Create(parent, winid, wxString(),
                                 pos, size,
                                 style | wxBU_NOTEXT | wxBU_EXACTFIT,
                                 validator, name);

include/wx/stc/stc.h

@@ -5239,7 +5239,7 @@ public:
 
     virtual void Remove(long from, long to) wxOVERRIDE
     {
-        Replace(from, to, "");
+        Replace(from, to, wxString());
     }
     virtual void Replace(long from, long to, const wxString& text) wxOVERRIDE
     {

include/wx/string.h

@@ -1617,7 +1617,7 @@ public:
     static wxString FromUTF8(const char *utf8)
     {
         if ( !utf8 || !wxStringOperations::IsValidUtf8String(utf8) )
-            return "";
+            return wxString();
 
         return FromImpl(wxStringImpl(utf8));
     }
@@ -1627,7 +1627,7 @@ public:
             return FromUTF8(utf8);
 
         if ( !utf8 || !wxStringOperations::IsValidUtf8String(utf8, len) )
-            return "";
+            return wxString();
 
         return FromImpl(wxStringImpl(utf8, len));
     }