Update fromAttribute for Arrays and Objects when no valid value is set

[f-santos]

For better understanding on the issue please check #722

This happens because JSON.parse('') will be fired and empty string is not a valid JSON.

My changes make an empty attribute to be converted to an empty array or an empty object what seems the expected behaviour in these cases not breaking internal methods.

Alternative solution

Change the default value to return undefined instead of the initial value as in my current solution.

Reference Issue

Fixes #722

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[f-santos]

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[justinfagnani]

In addition to the code change, we'll need a test that fails without it, and an update to the changelog.

[justinfagnani]

I think rather than this, which wouldn't fix the demo linked in the issue, we want to put a try/catch around the JSON.parse().

[felipeplets]

@justinfagnani @f-santos is an old user of mine which was deleted, so feel free to close this PR and consider #888 to fix the issue

[justinfagnani]

In Lit 2 we catch all JSON parse exceptions.