fix error on GATEWAY_MODEL_ZIG3 when no zigbee devices connected

[starkillerOG]

fix error on GATEWAY_MODEL_ZIG3 when no zigbee devices connected
see issue: home-assistant/core#51294

[rytilahti]

Nice and simple, can be merged when the lint checks pass!

[codecov-commenter]

Codecov Report

Merging #1065 (0c9b0c2) into master (6453bfe) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
- Coverage   75.42%   75.40%   -0.03%     
==========================================
  Files          73       73              
  Lines        8359     8367       +8     
  Branches      744      745       +1     
==========================================
+ Hits         6305     6309       +4     
- Misses       1870     1874       +4     
  Partials      184      184              

Impacted Files	Coverage Δ
miio/gateway/gateway.py	41.42% <0.00%> (-0.75%)	⬇️
miio/yeelight.py	87.90% <0.00%> (-0.31%)	⬇️
miio/airpurifier.py	97.02% <0.00%> (+0.01%)	⬆️
miio/vacuum.py	59.45% <0.00%> (+0.09%)	⬆️

---

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 6453bfe...0c9b0c2. Read the comment docs.

[starkillerOG]

@rytilahti this is ready to be merged.