Top-level parts with no chapter number appear as "null" in search #629
Description
Searching for "Type System" yields "4. The Type System" in the pop-up autocomplete as expected.
Searching for "null" yields several chapters that (intentionally) don't have chapter numbers, but they stringify as, e.g. "null Release Notes".
"null" should not produce these search results, and searching for "Release Notes" should suggest "Release Notes" rather than "null Release Notes".