Fix collapsed paper-dropdown-menu-light when set with empty value.

[mvtorres]

#190

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[mvtorres]

I signed it!

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On Feb 9, 2018, 4:25 PM -0500, googlebot ***@***.***>, wrote: Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 Please visit https://cla.developers.google.com/ to sign. Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks. • If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits. • If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot. The email used to register you as an authorized contributor must be the email used for the Git commit. • In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again. If the bot doesn't comment, it means it doesn't think anything has changed. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.

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[valdrinkoshi]

This LGTM but requires unit tests, mind adding those? e.g. select an item, then deselect it, and finally check that dropdownMenu.$.input.offsetHeight > 0.

[valdrinkoshi]

nit: this should work the same and do less checks, wdyt?

if (!this.value && this.value !== 0) {