Ignore a keydown that carries no key

Picking a saved login from Chrome's password autofill dispatches a plain
Event named "keydown", with no key on it. The shortcut listener passed it to
comboOf, which read the key's length and threw -- an uncaught TypeError in
the console on every sign-in. Harmless, since nothing was bound to it, but
it was noise that looks like a real fault.

comboOf now returns null for an event with no key, as it already does for a
bare modifier, so the listener stops there.

web/src/lib/keyboard.ts

@@ -136,6 +136,10 @@ const isMac = typeof navigator !== "undefined" && /Mac|iPhone|iPad/.test(navigat
 
 export function comboOf(e: KeyboardEvent): string | null {
   const key = e.key;
+  // Chrome's password autofill dispatches a plain Event named "keydown" when a
+  // saved login is picked: no key, nothing to match, and reading its length
+  // threw on every sign-in.
+  if (!key) return null;
   if (key === "Shift" || key === "Control" || key === "Alt" || key === "Meta") return null;
   const parts: string[] = [];
   const mod = isMac ? e.metaKey : e.ctrlKey;