Fixed a segfault when working with empty xccdf variable.
Put an empty string instead of NULL if xccdf_value_binding_get_value
returns NULL.
First, we do that before resolving variables conflict.
Second, we do that when adding variables to variable model.
New unit test, which tests possibility of segmentation fault when
working with empty XCCDF element "<variable/>".
Merge pull request #33 from jan-cerny/maint-1.0

Fixed a segfault when working with empty xccdf variable.