If you are invited to the meeting, then you should be professional and not be silent if you are the sole representative of your company. If your colleagues are not being professional, that will reflect poorly on them in the long run. Focus on how you can make the company (and you) look good and professional.
That said, when I am in this situation I will entertain the customer. You should do the same if you are comfortable with that approach.
Do not focus on the negative side of the situation. If you make bad comments about your colleagues, you look bad. If the customer leaves the call feeling abandoned, your company looks bad. If you don't say much until you offer to reschedule, you look bad. If your customer only remembers negative things, your company looks bad.
But if you engage the customer - mention that while you are waiting for your colleague(s), you can talk about "stuff" to pass the time, it reduces their focus on your colleague(s) absence. Ask how their day is going. Inquire if they have other concerns that are not within the focus of the meeting, take notes and pass them along to those in your company that can help. Ask how the weather is where they are. Ask if they like sports or cooking. Ask them if they know any good jokes!
This probably is not first time a meeting didn't go as planned for your customer. It won't be the last time. But engaging them and making a good impression on them not only helps your business look good, it helps you look good to your business. The customer will remember you, or else they will forget the problem with the meeting more easily. Every time you are engaged with a customer, it is an opportunity to sell them on your company. It is best not to waste that opportunity when you are given it, if you are able to take advantage of it.
I learned this from a similar situation I had early in my professional life. I was technical sales support and I showed up to a meeting with a customer where they had 20 representatives in the meeting, and my sales person was "running late." The room was small and hot, and I engaged in small talk to entertain them until we waited... to my astonishment, he walked in and after a 5 minute "introduction" to our company and without any warning to me, he said, "I bet you have a lot of technical questions, so I will leave you with our technical sales guy." And he walked out of the room! He didn't return for 45 minutes. So what did I do? I told them I was sales support, not sales. I simply said, "if you have any questions, ask me and I'll do my best." I got pounded with questions for the full 45 minutes. I didn't have all the answers but I wrote the ones down that I didn't know and promised to find the answers. When the sales person finally returned, they kicked him out so they could continue to ask me questions. I was exhausted and annoyed, but I acted professionally. After I returned and the customer gushed about my performance, I got a raise and a promotion. That was an extreme situation, and I learned from it how to handle calls where I was "support" and the "main event" didn't even show up.
As a note of caution, if you sense the customer is not responding well to your "entertainment" or you feel the awkwardness of your colleagues absence is growing, then try to gracefully suggest that you reschedule out of respect for their time. Keep the focus on their needs and desires. You are being respectful toward them for an unfortunate situation out of your control - that genuine concern and respect will, without doubt, make you and your company look good.
Other answers include talking to you boss about the problem - that is advisable, but your question is about how to handle the call, not how to deal with your colleagues unprofessional behavior. To me, "how to handle unprofessional colleagues" is a different question.
Don't waste time on what "could have been" or "should have been" - make the best of the time you have. And try to have fun along the way! Good luck!