Don't add aria-autocomplete when there are no completion sources

FIX: Avoid adding an `aria-autocomplete` attribute to the editor when there
are no active sources active.

See https://discuss.codemirror.net/t/avoiding-aria-autocomplete-list-when-no-autocompletion-providers/8313

src/state.ts

@@ -154,7 +154,7 @@ export class CompletionState {
 
   get tooltip(): Tooltip | null { return this.open ? this.open.tooltip : null }
 
-  get attrs() { return this.open ? this.open.attrs : baseAttrs }
+  get attrs() { return this.open ? this.open.attrs : this.active.length ? baseAttrs : noAttrs }
 }
 
 function sameResults(a: readonly ActiveSource[], b: readonly ActiveSource[]) {
@@ -172,6 +172,8 @@ const baseAttrs = {
   "aria-autocomplete": "list"
 }
 
+const noAttrs = {}
+
 function makeAttrs(id: string, selected: number) {
   let result: {[name: string]: string} = {
     "aria-autocomplete": "list",