Rollup merge of #57614 - GuillaumeGomez:fix-crate-filtering, r=QuietMisdreavus

[rustdoc] Fix crates filtering box not being filled

Currently, the filter crate box (at the left of the search input) is always empty. To get the number of keys of dictionary in JS, you need to call `Object.keys()` on it.

r? @QuietMisdreavus

src/librustdoc/html/static/main.js

@@ -2431,7 +2431,7 @@ if (!DOMTokenList.prototype.remove) {
             return;
         }
         var crates_text = [];
-        if (crates.length > 1) {
+        if (Object.keys(crates).length > 1) {
             for (var crate in crates) {
                 if (crates.hasOwnProperty(crate)) {
                     crates_text.push(crate);