If an article's citation_date value is empty, check to see if the Editor has supplied a value for the Issue Data's Year. If so, display that as the citation_date via the GS metadata template. Example code from Alec: 

{if $article->getDatePublished()}
    <meta ... value="{$article->getDatePublished()}"/>
{elseif $issue->getYear()}
    <meta ... value="{$issue->getYear()}"/>
{/if}