This is true for many designers because they work with customers do not know exactly what they want and/or have no clear idea of how a website can work for them and their business.
Content always goes first for all the reasons that have been explained here. But I want to stress the importance of what has been said.
Although you may not have all the content, the more you know about the business and the more content you have, the better.
What your customer (and you, as a developer) definately should know is the type of content that will be published and the categories and structure. It also helps to know if the content will be added regularly, the frequency of the updates, etc.
First, because the organization of that content will be necessary to do a proper navigation system as well as to adapt any search engine script that you use in the site.
A good organization system is essential to the success of one site, people don't like to click too much to get the information that they're looking for. And to do that, the classification system, the categories, etc. has to be very clear. So when you have enough content to know the type of articles the customer will write, and the length and other details, then it is so much easier to know if the system chosen for classifying the different articles is the right one.
Once the organization is clear, you can go to the design phase. Again, having at least some content helps because you will be able to see if the design really fits with the content or you should adapt to longer articles, or shorter paragraphs, etc.