fix(v-on): avoid events with empty keyCode (autocomplete)

[zupolgec]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:

Chromium browsers (and Safari) trigger a keydown/keyup event on autocomplete. An Event object without key or keyCode is generated.
Vue, when listening with modifiers, is triggering the callback even if the modifier key doesn't match the undefined keyCode of the event.

Current behaviour: https://codepen.io/zupolgec/pen/RwWRxdK
New behaviour: https://codepen.io/zupolgec/pen/qBONxMj

Try autocompleting the first field: in the first demo is counted, in the second demo is ignored.

See also same kind of issue on another JS library: alpinejs/alpine#389 (comment)

[posva]

This would indeed make the behavior consistent across browsers (e.g. Firefox doesn't trigger a keyup)!

Related commit: 29c348f