Fix broken empty address manager entries after restore on affected devices

[puddly]

The "empty" address manager entry differs between the CC253x and newer devices, causing networks created using the backup/restore tool to not permit new joins. I've added a NVRAM migration and have confirmed it fixes the problem with the CC2531 running both Z-Stack Home 1.2 and Z-Stack 3.0.2.

[codecov-commenter]

Codecov Report

Merging #92 (8510035) into dev (06af078) will increase coverage by 0.01%.
The diff coverage is 97.82%.

@@            Coverage Diff             @@
##              dev      #92      +/-   ##
==========================================
+ Coverage   98.75%   98.76%   +0.01%     
==========================================
  Files          44       44              
  Lines        3768     3801      +33     
==========================================
+ Hits         3721     3754      +33     
  Misses         47       47              

Impacted Files	Coverage Δ
zigpy_znp/znp/security.py	93.78% <92.85%> (-0.46%)	⬇️
zigpy_znp/api.py	98.17% <100.00%> (+<0.01%)	⬆️
zigpy_znp/const.py	100.00% <100.00%> (ø)
zigpy_znp/types/nvids.py	99.44% <100.00%> (+<0.01%)	⬆️
zigpy_znp/zigbee/application.py	96.89% <100.00%> (+0.35%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 06af078...8510035. Read the comment docs.