Database::makeList() : Handle NULL when building 'IN' clause

Database::makeList() : Handle NULL when building 'IN' clause

Amends to Database::makeList() to check if 'NULL' is present in array while
building IN clause. Presently a NULL in array intended to build IN clause, is
treated as a usual value. This creates
problem:https://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_in
Therefore null is appended at the end of the IN clause as " $field IS NULL"
rather than within the IN clause.
DatabaseSQLTest.php: Some test cases added for the modified behaviour.

Bug: T50853
Change-Id: Ic255c37e9702fd44d8173027ab8036ddc48f9d2e