SNSLiveEventDataListener: Add protection for empty elements in device descriptor

[Russell Taylor]

The SEQUOIA live stream has started to crash Mantid. The source has been traced to the ADARA device descriptor packet for the (just added) PulseMagnet device. This contains XML like:

<process_variable>
   <pv_name>PM_ADC</pv_name>
   <pv_id>1</pv_id>
   <pv_description>ADC capture</pv_description>
   <pv_type>integer</pv_type>
   <pv_units></pv_units>
</process_variable>

Notice that there's no text element contained in the pv_units element. In other cases where there's no unit the pv_units element is omitted entirely. In the above case we are getting back a null pointer when we try to get the text element and then dereferencing it.

[Russell Taylor]

Re #8564. Just some tidying up.

E.g. use empty() instead of size() == 0, ! instead of == NULL.

Changeset: 71ca580a65caaecc24aca05bdc89a7aefd6a3dfa

[Russell Taylor]

Re #8564. Guard against empty text elements in device descriptor pkts.

Changeset: b0b8410a3bbca9ee74b94ad4510f26b7fe760ce3

[Russell Taylor]

To test (needs to be at SNS):

Try to connect to the SEQUOIA live stream. Before the changes, this will crash Mantid (unless they've changed things upstream in the meantime), afterwards it should be all good and sample logs should be created correctly and some should have units (do a 'view sample logs').

[Ross Miller]

Merge remote branch 'origin/bugfix/8564_snslivelistener_devicedescriptor_crash'

Full changeset: ​a85e30f50f8fab82df4751518d9a1c399cf8ea5d

[Stuart Campbell]

This ticket has been transferred to github issue ​9408