Rename wxLocaleIdent::IsDefault() to IsEmpty()

We don't use empty wxLocaleIdent as default user locale and it doesn't
make much sense to do it, as the default locale is already accessible
via wxUILocale::GetDefault().

src/unix/uilocale.cpp

@@ -261,7 +261,7 @@ wxUILocaleImplUnix::~wxUILocaleImplUnix()
 void
 wxUILocaleImplUnix::Use()
 {
-    if ( m_locId.IsDefault() )
+    if ( m_locId.IsEmpty() )
     {
         // This is the default locale, it is already in use.
         return;