scd: Fix handling for Data Object with no data.

* scd/app-openpgp.c (get_cached_data): Return NULL for Data Object
with no data.

When GET_DATA returns no data with success (90 00), this routine
firstly returned buffer with length zero, and secondly (with cache)
returned NULL, which is inconsistent. Now, it returns NULL for both
cases.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>