Fix notices from an undefined post variable

[srtfisher]

Fixes notices from the shortcode when used on taxonomy pages (which do not have a $post set):

PHP message: Warning: Attempt to read property "ID" on null in /var/www/wp-content/plugins/polldaddy/polldaddy-shortcode.php on line 127

Changes proposed in this Pull Request:

Testing instructions:

Screenshot / Video

Proposed changelog entry for your changes:

[jgcaruso]

Hey there, thanks for the contribution and sorry for the delay in a response.

While the code change itself looks reasonable (and safer), would you mind providing testing instructions? I'm having trouble reproducing the original issue on the main branch. Pages set with a custom taxonomy seem to be working fine for me. Are you doing something differently? Is it on the taxonomy preview page and the full page content is being displayed instead of a summary?

My main concern here is I'd like to provide defaults for the values that are not having any values set (ex $unique_id)

Also, I don't think we want to introduce a composer.json, so would you mind excluding it from the PR? Thanks!