Backport "Fix: Correction of the siteRoot path" to LTS (#18957)

Backports #17297 to the LTS branch.

PR submitted by the release tooling.

Author: Kordyjan

scaladoc/src/dotty/tools/scaladoc/Scaladoc.scala

@@ -202,7 +202,7 @@ object Scaladoc:
         classpath.get,
         bootclasspath.get,
         destFile,
-        siteRoot.nonDefault,
+        Option(siteRoot.withDefault(siteRoot.default)),
         projectVersion.nonDefault,
         projectLogo.nonDefault,
         projectFooter.nonDefault,