Fixing format dependent header display

dependency-tree.ts

@@ -98,7 +98,9 @@ const constructDepTree = (packageName: string, context: PackageLockDependencies,
 }
 
 const displayDepTree = (format: Format) => {
-    console.log(format.header);
+    if (format.header) {
+        console.log(format.header);
+    }
     return (filter: (node: PackageLockTree, parents: string[]) => boolean) =>
         (tree: PackageLockTree) => {
             const iter = (tree: PackageLockTree, stack: string[] = []) => {