I would like to replace a couple of electrical outlets in my house with ones that have USB sockets. My concern is that they will continually draw power, even when I don't have a phone plugged in to them
It is very unlikely that they won't. Nearly all AC to DC converters, by design, draw a very small idle current when you aren't charging any devices. Cheaper designs consume more while some good designs consume less, but unless there's a physical switch on the USB socket it will always consume some power while waiting for a device to be plugged in.
These are new sockets to the market, and unless you have a compelling reason to avoid a plug in USB charger I would recommend holding off anyway. Not only do you have the continuous current draw (which, honestly, is going to be less then $5.00 per year even if the device is terribly inefficient) but there's the likelihood of the socket failing, and it's not nearly as easily or cheaply replaced as a plug in charger. Further it may not charge all devices at their full current. While the USB association has suggested designs for high power charging (above 500mA), many manufacturers (notably, Apple) still use their own scheme for detecting 1A and 2A charging currents.
Most phones and PDAs will only charge at 1A or greater when connected to their own charger, and at the slower 500mA rate when they can't detect their own charger.
There are probably some applications and situations where it might be useful, for those prone to losing their chargers, for instance, but there are many reasons to reconsider using them.
This product has little spring-loaded doors on the USB ports that completely shut off the power supply when they're closed.
You will most likely have to find the results of a test of the specific model you are considering purchasing. It depends entirely on the design of the power supply built into the outlet, and there is a lot of room for variation.
You could conduct the test yourself by buying one such outlet, wiring it temporarily to a plug, and using a plug-in power meter or clamp meter to measure the current/power draw. Make sure the meter has enough resolution to report this very small load.
Vampire power on modern devices is negligible. This site did a test, they couldn't get a reading with their equipment with any single charger. They had to plug a bunch in. 6 devices plus the led light on the power strip was 30 cents/yr.
The combined total vampire power draw of this power bar, an iPhone 6 charger, an iPad Air charger, a MacBook Air (2013) charger, a Surface Pro 2 charger, a Samsung Chromebook charger, and a Nexus 7 charger read 0.3 watts.