stop for loop after genesis block

prevHash == undefined for the genesis block

backend/src/routes.ts

@@ -567,7 +567,7 @@ class Routes {
       }
 
       let nextHash = startFromHash;
-      for (let i = 0; i < 10; i++) {
+      for (let i = 0; i < 10 && nextHash; i++) {
         const localBlock = blocks.getBlocks().find((b) => b.id === nextHash);
         if (localBlock) {
           returnBlocks.push(localBlock);