text: Title: A content in `extra_navbar` is no longer shown after updating to 0.15.0
  Body: ### Describe the bug
  
  **context**
  A content in `extra_navbar` for `html` in `_config.yml` is no longer shown after updating to  0.15.0
  
  **expectation**
  I expected the content to be shown.
  
  **bug**
  No error message.
  
  
  ### Reproduce the bug
  
  Update to 0.15.0 and build the book.
  
  ### List your environment
  
  Jupyter Book      : 0.15.0
  External ToC      : 0.3.1 
  MyST-Parser       : 0.18.1
  MyST-NB           : 0.17.1
  Sphinx Book Theme : 1.0.0
  Jupyter-Cache     : 0.5.0  
  NbClient          : 0.5.13