Fix bug with connection string generator

When setting up BVCMS for the first time, I found an apparent bug with the connection string generator.
The null coalescing operator for connection string "datasource" was causing an incorrect connection string to be made (datasource would always be an empty string). 
Changing to an if statement and using string.isnullorempty seemed to solve the problem.

(Ignore changes I made to ConnectionStrings.config)