Basically, for pages that may not have a description or tags set, is it better to have
<meta name="description" content="" /> <meta name="keywords" content="" />
or just skip the tags altogether for those pages that don't have the information set?
FYI, meta tags have no effects on rankings so no meta tags is the same as empty meta tags is the same as full meta tags.
Having said that, empty meta tags are the same as no meta tags. Either way you are not providing search engines or any other crawler any information normally provided in those tags. If I had to choose one or the other I would simply omit them as it means a (slightly) smaller file size and less parsing for crawlers.
There is no benefit to having an empty meta description or meta keywords. They take up a few bytes in the page source. You might as well leave them out.
On the flip side, it shouldn't really hurt when those fields are empty compared to not having them. If you CMS requires that they are there and you don't want to write content for them, it would be OK to leave them empty. Especially the meta keywords, they are not used at all by Google anymore. Writing a good meta description is generally a good idea because it often gets shown as the snippet in the search results. Writing a good meta description can help pull in users that see your listing.