calcurse-caldav: Avoid empty multiget

When there are no new objects to pull from the server, skip the import
routine instead of sending a query that requests zero items.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

contrib/caldav/calcurse-caldav.py

@@ -298,6 +298,9 @@ def pull_objects(conn, syncdb, etagdict):
         elif etagdict[href] != syncdb[href][0]:
             modified.add(href)
 
+    if not missing and not modified:
+        return 0
+
     # Download and import new objects from the server.
     body = ('<?xml version="1.0" encoding="utf-8" ?>'
             '<C:calendar-multiget xmlns:D="DAV:" '