Fix crash if server does not provide DiscoveryUrls

The OPC UA specification does not forbid that a server does not offer DiscoveryUrls. This can lead to a crash when calling the function application_to_strings().

Check DiscoveryUrls for None or empty before iterating to avoid the crash described in #1127.

asyncua/tools.py

@@ -525,6 +525,7 @@ def application_to_strings(app):
     for (n, v) in optionals:
         if v:
             result.append((n, v))
+    if app.DiscoveryUrls:
         for url in app.DiscoveryUrls:
             result.append(("Discovery URL", url))
     return result  # ['{}: {}'.format(n, v) for (n, v) in result]