This PR enhances the test coverage for the index.Digester by adding the following test scenarios:

• Ensures WithIndex correctly handles a nil input.
• Verifies that multiple WithIndex options are processed, with the last one applied.
• Confirms that Digester.Add can handle empty keys and values.
• Ensures Digester.Delete gracefully handles the deletion of non-existent keys.
• Verifies that Digester.Index returns an empty copy for an empty internal index.
• Checks that the Len method returns 0 for a new, empty Digester.
• Verifies that the String method returns an empty string for an empty Digester.