FileSetConfig works with empty set of roots

Closes #5139

Author: matklad

crates/vfs/src/file_set.rs

@@ -69,7 +69,7 @@ impl FileSetConfig {
             Ok(it) => it,
             Err(it) => it.saturating_sub(1),
         };
-        if path.starts_with(&self.roots[idx].0) {
+        if !self.roots.is_empty() && path.starts_with(&self.roots[idx].0) {
             self.roots[idx].1
         } else {
             self.len() - 1