Fix infinite loop causing memory leak in GH-1519

[joshbode]

I don't know the codebase/protocol well enough to know whether we can always expect there to be at least one value in the iterator, or whether we need to have a fallback and a check, e.g.

head = next(iter(results), None)
if head is None:
    break

[oroulet]

I reverted change in that other MR: https://github.com/FreeOpcUa/opcua-asyncio/pull/1534. Thank for you help @joshbode !

[joshbode]

Sounds good, @oroulet :)