[network-data] add nullptr check in Iterate() (#5359)
[network-data] add nullptr check in Iterate() (#5359)


bukepo/openthread,jwhui/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,srickardti/openthread,bukepo/openthread,chshu/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,bukepo/openthread,srickardti/openthread,openthread/openthread,bukepo/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,openthread/openthread,chshu/openthread,chshu/openthread