webdriver: fix expected key code for "Unidentified" keys

Per the UI Events specification
(https://w3c.github.io/uievents-code/#key-legacy), keys such as
\u00e0 (i.e., à) should have a code of "Unidentified".  Some tests
in key.py expect an empty value, which seems to be an oversight.

webdriver/tests/perform_actions/key_events.py

@@ -105,11 +105,11 @@ def test_non_printable_key_sends_events(session, key_reporter, key_chain, key, e
     ("a", "KeyA",),
     (u"\"", "Quote"),
     (u",", "Comma"),
-    (u"\u00E0", ""),
-    (u"\u0416", ""),
+    (u"\u00E0", "Unidentified"),
+    (u"\u0416", "Unidentified"),
     (u"@", "Digit2"),
-    (u"\u2603", ""),
-    (u"\uF6C2", ""),  # PUA
+    (u"\u2603", "Unidentified"),
+    (u"\uF6C2", "Unidentified"),  # PUA
 ])
 def test_printable_key_sends_correct_events(session, key_reporter, key_chain, value, code):
     key_chain \