fix(core): Make flattenRoutePath return a valid module

flattenRoutePath method was returning an object instead of a module when path property was an empty string

closes #15332

Author: sebastiantreu

packages/core/router/utils/flatten-route-paths.util.ts

@@ -14,7 +14,7 @@ export function flattenRoutePaths(routes: Routes) {
     if (item.children) {
       const childrenRef = item.children as Routes;
       childrenRef.forEach(child => {
-        if (!isString(child) && child.path) {
+        if (!isString(child) && isString(child.path)) {
           child.path = normalizePath(
             normalizePath(item.path) + normalizePath(child.path),
           );

packages/core/test/router/utils/flat-routes.spec.ts

@@ -17,6 +17,10 @@ describe('flattenRoutePaths', () => {
     @Module({})
     class ChildModule4 {}
     @Module({})
+    class ChildModule5 {}
+    @Module({})
+    class ChildParentPathModule {}
+    @Module({})
     class ParentChildModule {}
     @Module({})
     class ChildChildModule2 {}
@@ -70,6 +74,16 @@ describe('flattenRoutePaths', () => {
               ChildModule4,
             ],
           },
+          {
+            path: 'child3',
+            children: [
+              {
+                path: '',
+                module: ChildModule5,
+                children: [{ path: 'child', module: ChildParentPathModule }],
+              },
+            ],
+          },
         ],
       },
       { path: '/v1', children: [AuthModule, CatsModule, DogsModule] },
@@ -91,6 +105,8 @@ describe('flattenRoutePaths', () => {
       },
       { path: '/parent/child2', module: ChildModule4 },
       { path: '/parent/child2/child', module: ChildModule3 },
+      { path: '/parent/child3', module: ChildModule5 },
+      { path: '/parent/child3/child', module: ChildParentPathModule },
       { path: '/v1', module: AuthModule },
       { path: '/v1', module: CatsModule },
       { path: '/v1', module: DogsModule },