The requirement for the context on the deploy page is just an artifact of that process. Simply put "/" in there, and it will deploy fine for you. This field overrides the sun-web.xml file. If you deploy it from the command line, and don't specify the context root option, it will use one in sun-web.xml.
Using "/" as the context root from the deployment screen, I've never had a problem deploying a root web app.
If you like, you can submit a RFE to ask that the admin console better handle an empty context root field in the console.
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