tests/qapi-schema: Cover 'not' condition with empty argument

We flag this, but the error message is bogus:

    bad-if-not.json:2: 'if' condition [] of struct is useless

The next commit will fix it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210908045428.2689093-5-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

tests/qapi-schema/bad-if-not.err

@@ -0,0 +1,2 @@
+bad-if-not.json: In struct 'TestIfStruct':
+bad-if-not.json:2: 'if' condition [] of struct is useless

tests/qapi-schema/bad-if-not.json

@@ -0,0 +1,3 @@
+# check 'if not' with empy argument
+{ 'struct': 'TestIfStruct', 'data': { 'foo': 'int' },
+  'if': { 'not': '' } }

tests/qapi-schema/meson.build

@@ -43,6 +43,7 @@ schemas = [
   'bad-if-key.json',
   'bad-if-keys.json',
   'bad-if-list.json',
+  'bad-if-not.json',
   'bad-type-bool.json',
   'bad-type-dict.json',
   'bad-type-int.json',