BaseTERule: Fix conditional_block property.

In apol, unconditional TE rules were shown as having no conditional but
as being in the False block.  Since the _conditional_block attribute is
bint, it will never be None.  Use the _conditional attribute to
determine if the rule is conditional.

setools/policyrep/terule.pxi

@@ -80,7 +80,7 @@ cdef class BaseTERule(PolicyRule):
             If the rule is here, this property is False
         }
         """
-        if self._conditional_block is None:
+        if self._conditional is None:
             raise RuleNotConditional
         else:
             return self._conditional_block