Fix JS to match Rust definition of optional field.

Author: kpreid

src/librustdoc/html/static/js/main.js

@@ -1013,7 +1013,7 @@ function preLoadCss(cssUrl) {
         // same heading are grouped.
         const cratesGroupedByHeading = new Map();
         for (const entry of window.ALL_CRATES) {
-            const heading = entry.h;
+            const heading = entry.h || "";
             const crateName = entry.c;
             let group = cratesGroupedByHeading.get(heading);
             if (group === undefined) {

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

@@ -403,7 +403,7 @@ declare namespace rustdoc {
          * 
          * May be empty to specify the first, primary heading.
          */
-        h: string,
+        h?: string,
         /** Crate name. */
         c: string,
     }