SELECT * FROM Table WHERE (FirstName = '' OR FirstName = @FirstName) AND (LastName = '' OR LastName = @LastName

var sql = "SELECT * FROM Table WHERE 1 = 1 /* Just to not care about if the where clause is needed */";

if (!String.IsNullOrWhiteSpace(FirstName))
{
    sql += " AND FirstName = @FirstName ";
}

if (!String.IsNullOrWhiteSpace(LastName))
{
    sql += " AND LastName= @LastName";
}