rustdoc-search: fix where clause highlight with dup generics

Essentially, the unifier only adds generics to a parameter when
a part of its where clause is highlighted. To make that work,
the formatter can ignore the empties.

Author: notriddle

src/librustdoc/html/static/js/search.js

@@ -1574,7 +1574,9 @@ function initSearch(rawSearchIndex) {
                     for (const nested of fnType.generics) {
                         writeFn(nested, where);
                     }
-                    whereClause.set(fnParamNames[-1 - fnType.id], where);
+                    if (where.length > 0) {
+                        whereClause.set(fnParamNames[-1 - fnType.id], where);
+                    }
                 } else {
                     if (fnType.ty === TY_PRIMITIVE) {
                         if (fnType.id === typeNameIdOfArray || fnType.id === typeNameIdOfSlice ||

tests/rustdoc-js-std/vec-type-signatures.js

@@ -19,4 +19,16 @@ const EXPECTED = [
             { 'path': 'std::vec::IntoIter', 'name': 'next_chunk' },
         ],
     },
+    {
+        'query': 'vec<Allocator> -> Box<[T]>',
+        'others': [
+            {
+                'path': 'std::boxed::Box',
+                'name': 'from',
+                'displayType': '`Vec`<T, `A`> -> `Box`<`[T]`, A>',
+                'displayMappedNames': `T = T`,
+                'displayWhereClause': 'A: `Allocator`',
+            },
+        ],
+    },
 ];