Allow NULL as data in storage client if length is 0

NULL data should not be treated as an invalid input if the length is
also 0 in set/get calls of the secure storage client. This can be used
to create an empty storage or check the existence of a storage uid.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I21c3a261913d5c28ca11cf12755d8b1b5570ed7f
1 file changed