fix panic when running pagerank on empty graph

cozo-core/src/fixed_rule/algos/pagerank.rs

@@ -40,6 +40,10 @@ impl FixedRule for PageRank {
 
         let (graph, indices, _) = edges.as_directed_graph(undirected)?;
 
+        if indices.is_empty() {
+            return Ok(())
+        }
+
         let (ranks, _n_run, _) = page_rank(
             &graph,
             PageRankConfig::new(iterations, epsilon as f64, theta),