ITS: Increase test coverage

Add the following test cases:
* Performing a get with a null data_length pointer and a data size
  greater than 0 returns a failure.
* Setting an asset with null data pointer and data length greater
  than 0 returns a failure.
* Getting asset info with null pointer returns a failure.
* Performing a get with null data pointer and data size greater than
  0 returns a failure.

Signed-off-by: Awadhy Mohammed <awadhy.mohammed@arm.com>
Change-Id: I7a42bc9d46c7b14ff414311f89fc7ca4b6990394
4 files changed