tdf#146250 fix SvxHtmlOptions::IsDefaultTextEncoding

regression from
    commit da9bba7cc3
    Author: Noel Grandin <noel.grandin@collabora.co.uk>
    Date:   Mon Jul 26 10:10:17 2021 +0200
    use officecfg for SvxHtmlOptions

spotted by himajin100000

Change-Id: Id237f7c4d6d816539141de97a19add0a993d92ba
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/126902
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

svtools/source/config/htmlcfg.cxx

@@ -90,7 +90,8 @@ bool IsPrintLayoutExtension()
 bool IsDefaultTextEncoding()
 {
     std::optional<sal_Int32> x = officecfg::Office::Common::Filter::HTML::Export::Encoding::get();
-    return bool(x);
+    // if we have a value, then the text encoding is not default
+    return !bool(x);
 }
 rtl_TextEncoding GetTextEncoding()
 {