Fix type of template getter in element-mixin.html

[bro314]

Fix type of template getter in element-mixin.html

Fixes #5443

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[TimvdLippe]

Pending approval by @rictic

[arthurevans]

Just to note, the docs currently recommend returning null in one specific (probably very rare) circumstance--if you're extending an element with a template, but you don't want your element to have one. https://polymer-library.polymer-project.org/3.0/docs/devguide/dom-template#elements-with-no-shadow-dom

If the strict template changes changed this behavior, we should update the docs, but pinging @kevinpschaaf here in case he has input.

[rictic]

As I read it, this method can return either undefined or null.

undefined if the mixed-in element doesn't have a template, either as a dom-module or inline.

null if the mixed-in element explicitly declares a null template, as we recommend that you do if it's an element with no template (like e.g. iron-ajax)

I'd prefer:

{string|!HTMLTemplateElement|null|undefined}

[bro314]

Done. PTAL

[bro314]

HTMLTemplateElement without ! implicitly means nullable, so I think just removing the ! from the setter parameter type comment is the way to go. Changed both to {HTMLTemplateElement|string}. PTAL