In SQL Server, you can create a "singleton" table (a table with one row) like this:
CREATE TABLE configurationData ( id AS 1 PRIMARY KEY , properties NVARCHAR(MAX) NOT NULL )
The primary key is a computed column with a constant value. Consequently, any attempt to insert a second row into the table will be greeted with a constraint violation error.
Note that this method only ensures that there cannot be two or more rows in the table -- it does not prevent the table from being empty.