interception route: fix route groups breaking the referrer computation (

#49602)

This PR fixes a bug during path normalisation where `/(foo)/` would
break because it would return `` per the function logic. A simple fix is
to just ensure we always return a `/` as the default if the
normalisation returns an empty string.

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link NEXT-1118

packages/next/src/client/components/router-reducer/compute-changed-path.test.ts

@@ -0,0 +1,60 @@
+import { computeChangedPath } from './compute-changed-path'
+
+describe('computeChangedPath', () => {
+  it('should return the correct path', () => {
+    expect(
+      computeChangedPath(
+        [
+          '',
+          {
+            children: [
+              '(marketing)',
+              {
+                children: ['__PAGE__', {}],
+                modal: [
+                  '(...)stats',
+                  {
+                    children: [
+                      ['key', 'github', 'd'],
+                      {
+                        children: ['__PAGE__', {}],
+                      },
+                    ],
+                  },
+                ],
+              },
+            ],
+          },
+          undefined,
+          undefined,
+          true,
+        ],
+        [
+          '',
+          {
+            children: [
+              '(marketing)',
+              {
+                children: ['__PAGE__', {}],
+                modal: [
+                  '(...)stats',
+                  {
+                    children: [
+                      ['key', 'github', 'd'],
+                      {
+                        children: ['__PAGE__', {}],
+                      },
+                    ],
+                  },
+                ],
+              },
+            ],
+          },
+          undefined,
+          undefined,
+          true,
+        ]
+      )
+    ).toBe('/')
+  })
+})

packages/next/src/client/components/router-reducer/compute-changed-path.ts

@@ -11,13 +11,18 @@ const segmentToPathname = (segment: Segment): string => {
 }
 
 function normalizePathname(pathname: string): string {
-  return pathname.split('/').reduce((acc, segment) => {
-    if (segment === '' || (segment.startsWith('(') && segment.endsWith(')'))) {
-      return acc
-    }
+  return (
+    pathname.split('/').reduce((acc, segment) => {
+      if (
+        segment === '' ||
+        (segment.startsWith('(') && segment.endsWith(')'))
+      ) {
+        return acc
+      }
 
-    return `${acc}/${segment}`
-  }, '')
+      return `${acc}/${segment}`
+    }, '') || '/'
+  )
 }
 
 export function extractPathFromFlightRouterState(