SqlServerBatchRunner handling of null parameters

This pull request contains a small change to address an issue I encountered with the SqlServerBatchRunner when dealing with string parameters in expressions.  Where a string parameter in the expression has a null value, the SqlServerBatchRunner currently adds Parameters to the ADO.Net Command objects with null values rather than DbNull.Value which causes an exception to be thrown in some cases.

The type of query expression that triggered this problem is of the type:

 db.Users.Where(u => u.EmailAddress.EndsWith(emailDomain) && (string.IsNullOrEmpty(optionalComparisonString) || u.AvatarType == optionalComparisonString))
.Delete();

The query can alternatively be rewritten in pieces to avoid this problem... (ie add the filter expression only where the optional parameter is non-null).. however when there are a number of these kinds of parameters it is more convenient to use the form above.

I have added some tests to cover this.