koops: fix json output if there are no modules

Closes #113.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #114 from abrt/gh113-json-no-modules

koops: fix json output if there are no modules

Closes #113.

Signed-off-by: Martin Milata <mmilata@redhat.com>