Fix initial context for Cody Web on the repository page (#63497)

Closes
https://linear.app/sourcegraph/issue/SRCH-620/cody-web-has-incorrect-initial-context-on-blank-repository-page

## Test plan
- Go to the repository root and open Cody Web, it should have only
repository as the initial context

client/web/src/cody/sidebar/new-cody-sidebar/NewCodySidebarWebChat.tsx

@@ -35,7 +35,7 @@ export const NewCodySidebarWebChat: FC<NewCodySidebarWebChatProps> = memo(functi
     const contextInfo = useMemo(
         () => ({
             repositories: [repository],
-            fileURL: `/${filePath}`,
+            fileURL: filePath ? `/${filePath}` : undefined,
         }),
         [repository, filePath]
     )