Fix a crash in 'Fix handling empty vm_type in Store.last_buildroot' when last_buildroot is empty

[dmach]

No description provided.

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.02% 🎉

Comparison is base (f788db0) 30.36% compared to head (ee725ff) 30.38%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1414      +/-   ##
==========================================
+ Coverage   30.36%   30.38%   +0.02%     
==========================================
  Files          49       49              
  Lines       17694    17698       +4     
==========================================
+ Hits         5372     5377       +5     
+ Misses      12322    12321       -1     

Files Changed	Coverage Δ
osc/conf.py	52.67% <0.00%> (ø)
osc/store.py	84.52% <100.00%> (+0.62%)	⬆️

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