fix: make sure the resulting array size is equal to the number of binders

The following code relies on this property
```lean
       for uid in scope.varUIds, x in xs do
          sectionFVars := sectionFVars.insert uid x
```

src/Lean/Elab/Command.lean

@@ -306,7 +306,7 @@ def getBracketedBinderIds : Syntax → Array Name
   | `(bracketedBinder|($ids* $[: $ty?]? $(annot?)?)) => ids.map Syntax.getId
   | `(bracketedBinder|{$ids* $[: $ty?]?})            => ids.map Syntax.getId
   | `(bracketedBinder|[$id : $ty])                   => #[id.getId]
-  | `(bracketedBinder|[$ty])                         => #[]
+  | `(bracketedBinder|[$ty])                         => #[Name.anonymous]
   | _                                                => #[]
 
 private def mkTermContext (ctx : Context) (s : State) (declName? : Option Name) : Term.Context := do

tests/lean/run/secVarBug.lean

@@ -0,0 +1,8 @@
+variable {α} [ToString α] (n : Nat)
+
+local macro "foo" : term => `(n)
+
+def f :=
+  foo
+
+#check f 10