Mark date inputs as valid when (partially) cleared (#1154)

Chrome 83 has removed the clear button from date inputs, which means that there's now no way to clear a date field and have its validity state correctly updated.  Since the `oninput` event fired when the first date component is cleared provides an empty string as the new value, we can detect this and override `validity.badInput` to immediately update our validity state in order to avoid leaving the field permanently flagged as invalid.

Fixes #1153

addon/components/paper-input.js

@@ -191,7 +191,14 @@ export default Component.extend(FocusableMixin, ColorMixin, ChildMixin, Validati
       });
       this.growTextarea();
       let inputElement = this.element.querySelector('input');
-      this.set('isNativeInvalid', inputElement && inputElement.validity && inputElement.validity.badInput);
+      let isNativeInvalid = inputElement && inputElement.validity && inputElement.validity.badInput;
+      if (this.type === 'date' && e.target.value === '') {
+        // Chrome doesn't fire the onInput event when clearing the second and third date components.
+        // This means that we won't see another event when badInput becomes false if the user is clearing
+        // the date field.  The reported value is empty, though, so we can already mark it as valid.
+        isNativeInvalid = false;
+      }
+      this.set('isNativeInvalid', isNativeInvalid);
       this.notifyValidityChange();
     },