Fixing the Key decode issue for multi-events

[shrohilla]

We were retrieving the key value for the case of multi-event from the already retrieved data, due to which customer was facing the crash in their basic scenario.
Customer has raised this issue multiple times -- Azure/azure-functions-kafka-extension#388

[codecov]

Codecov Report

Merging #146 (87d0ee5) into dev (aff8ce3) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #146      +/-   ##
==========================================
- Coverage   90.13%   90.06%   -0.07%     
==========================================
  Files          48       48              
  Lines        2918     2918              
  Branches      385      385              
==========================================
- Hits         2630     2628       -2     
- Misses        213      214       +1     
- Partials       75       76       +1     

Flag	Coverage Δ
unittests	90.02% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
azure/functions/kafka.py	91.79% <ø> (ø)
azure/functions/meta.py	77.41% <0.00%> (-1.08%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.