Trim tag input + don't allow whitespace tags

[srindom]

As per Philip's comment: https://github.com/medusajs/admin/pull/368#pullrequestreview-883041589

When adding tags the new string value should be trimmed - also if the resulting string is empty we should ignore the update; i.e. don't add the string to the values array and reset the input field's content.

This is the place to add changes: https://github.com/medusajs/admin/blob/master/src/components/molecules/tag-input/index.tsx#L46