Parse missing agent manifests as empty

[narrieta]

Old versions of the HGAP can sometimes produce goal states with missing agent manifests, which causes a parsing error while fetching the goal state. The fix parses that as an empty list.

[codecov]

Codecov Report

Merging #2615 (7e39ec8) into release-2.8.0.0 (fcc81a1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                 Coverage Diff                 @@
##           release-2.8.0.0    #2615      +/-   ##
===================================================
+ Coverage            72.18%   72.20%   +0.01%     
===================================================
  Files                  102      102              
  Lines                15391    15393       +2     
  Branches              2442     2443       +1     
===================================================
+ Hits                 11110    11114       +4     
+ Misses                3793     3792       -1     
+ Partials               488      487       -1     

Impacted Files	Coverage Δ
...protocol/extensions_goal_state_from_vm_settings.py	83.45% <100.00%> (+0.12%)	⬆️
azurelinuxagent/ga/collect_telemetry_events.py	92.12% <0.00%> (+0.68%)	⬆️

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