qapi: Fix to reject empty union base gracefully

Common Python pitfall: 'assert base_members' fires on [] in addition
to None.  Correct to 'assert base_members is not None'.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-10-git-send-email-armbru@redhat.com>

scripts/qapi.py

@@ -735,7 +735,7 @@ def check_union(expr, info):
             raise QAPISemError(info, "Flat union '%s' must have a base"
                                % name)
         base_members = find_base_members(base)
-        assert base_members
+        assert base_members is not None
 
         # The value of member 'discriminator' must name a non-optional
         # member of the base struct.

tests/qapi-schema/union-base-empty.err

-Traceback (most recent call last):
-  File "tests/qapi-schema/test-qapi.py", line 56, in <module>
-    schema = QAPISchema(sys.argv[1])
-  File "scripts/qapi.py", line 1487, in __init__
-    self.exprs = check_exprs(parser.exprs)
-  File "scripts/qapi.py", line 921, in check_exprs
-    check_union(expr, info)
-  File "scripts/qapi.py", line 738, in check_union
-    assert base_members
-AssertionError
+tests/qapi-schema/union-base-empty.json:5: Discriminator 'type' is not a member of base struct 'Empty'