NIFI-10305 Fix network and cpu data in c2 heartbeat

[ferencerdei]

Summary

This PR fix the missing network information node in C2 Heartbeat response which happens when the running env has multiple network interfaces and some doesn't have IP assigned to it. Also removing the invalid negative value from the CPU Load average when the running env does not support that command.

NIFI-10305

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[bejancsaba]

+1, thank you for making this part more robust, we will need to extend the system tests when we get there to have some proper coverage on this as well