Avoid "Cannot read properties of undefined (reading 'length')"

[tayles]

On a fresh install of the latest docusaurus (v2.4.3) and docusaurus-lunr-search (v3.0.0) the following error is thrown in non-production mode:

TypeError: Cannot read properties of undefined (reading 'length')
    at eval (webpack-internal:///./node_modules/docusaurus-lunr-search/src/theme/SearchBar/index.jsx:17:1504)

I believe this is caused by the changes in 426e3131d7, where in development mode the searchDocFile (from getSearchDoc()) is just an empty array.