Protect against DeviceAttribute without .value

[mrosanes]

DeviceAttribute is not providing .value in some cases.
This has been seen using PyTango 9.3.0 when trying to read empty
spectrums.

Protect against DeviceAttribute not providing .value.

[mrosanes]

Read empty spectrum fails on taurus using pytango 9.3.0

Root problem: PyTango 9.3.0 also fails on reading empty spectrums. Most probably this is already introduced in Tango C/C++.

This failure has been found when executing Sardana spock using PyTango 9.3.0

To reproduce the problem easily without using Sardana from ipython do (in a system with PyTango 9.3.0):

import tango
import taurus
d = tango.DeviceProxy("sys/tg_test/1")
d.short_spectrum = []
taurus.Attribute("sys/tg_test/1/short_spectrum").read()

This problem is related with tango-controls/pytango#271

This PR adds a protection in Taurus when DeviceAttribute does not provide .value

[cpascual]

Thanks,

Note that the fix of tango-controls/pytango#271 (in tango-controls/pytango#273) makes Tango C++ and PyTango differ in this respect, which is probably not a good permanent solution, so I would expect that the either the old (pre-9.3) behaviour is restored in Tango or that PyTango would end adapting to the new (backwards-incompatible) behaviour.

In either case, the changes in this PR would protect us.