search.js: displayTypeSignature should default to undef, not null

this is because tester.js iterates over
the keys of each ResultObject.

Author: lolbinarycat

src/librustdoc/html/static/js/rustdoc.d.ts

@@ -257,7 +257,7 @@ declare namespace rustdoc {
         ty: number,
         type?: FunctionSearchType,
         paramNames?: string[],
-        displayTypeSignature: Promise<rustdoc.DisplayTypeSignature> | null,
+        displayTypeSignature: Promise<rustdoc.DisplayTypeSignature> | undefined,
         item: Row,
         dontValidate?: boolean,
     }