History panel: Do not crash when receiving invalid JSON

[matthiask]

Fixes #1403

[codecov]

Codecov Report

Merging #1415 (cf01994) into master (1181f2d) will increase coverage by 0.78%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1415      +/-   ##
==========================================
+ Coverage   86.97%   87.76%   +0.78%     
==========================================
  Files          29       29              
  Lines        1574     1577       +3     
  Branches      220      220              
==========================================
+ Hits         1369     1384      +15     
+ Misses        150      142       -8     
+ Partials       55       51       -4     

Impacted Files	Coverage Δ
debug_toolbar/panels/history/panel.py	95.83% <100.00%> (+6.94%)	⬆️
debug_toolbar/middleware.py	96.66% <0.00%> (+1.66%)	⬆️
debug_toolbar/toolbar.py	91.25% <0.00%> (+2.50%)	⬆️
debug_toolbar/panels/profiling.py	89.28% <0.00%> (+2.67%)	⬆️
debug_toolbar/panels/headers.py	100.00% <0.00%> (+4.54%)	⬆️
debug_toolbar/panels/templates/views.py	67.44% <0.00%> (+4.65%)	⬆️

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