qapi/introspect.py: Always define all 'extra' dict keys

This mimics how a typed object works, where 'if' and 'comment' are
always set, regardless of if they have a value set or not.

It is safe to do this because of the way that _tree_to_qlit processes
these values (using dict.get with a default of None), resulting in no
change of output from _tree_to_qlit. There are no other users of this
data.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-9-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/introspect.py

@@ -27,11 +27,10 @@
 
 
 def _make_tree(obj, ifcond, comment=None):
-    extra = {}
-    if ifcond:
-        extra['if'] = ifcond
-    if comment:
-        extra['comment'] = comment
+    extra = {
+        'if': ifcond,
+        'comment': comment
+    }
     return (obj, extra)