Fix issue in ZmqSubscriber that can cause a segfault

[eflumerf]

No description provided.

[dingp]

Found an interesting post about different ways of returning empty string.

https://stackoverflow.com/questions/26587110/returning-an-empty-string-efficient-way-in-c

Looks like return {} might be better.

[eflumerf]

Interesting, if almost certainly a micro-optimization in this case, since ZmqSubscriber::connect_for_sends should only be called 1/s in the worst case where the other end of the connection is never published to the Connectivity Service

[dingp]

LGTM

[bieryAtFnal]

My tests have all run fine so far, but I haven't yet been able to find where the code that calls connect_for_receives() checks whether the returned string is empty...

[eflumerf]

It doesn't, at the moment (it would be in NetworkManager::create_receiver). While debugging this without the change in iomanager, there was an exception raised from that code when it tried to decode the empty string as a ZMQ connection string and failed...

[dingp]

Going to merge this PR, and create a new tag to be used for the third candidate release for v4.1.0 (rc-v4.1.0-3 and fd-v4.1.0-c3).