Fix for acking tombstone messages

[patkivikram]

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Description

This fixes the issue of not acking tombstone messages on a topic processed by the agent
#439

[codecov-io]

Codecov Report

Merging #442 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #442   +/-   ##
======================================
  Coverage    99.9%   99.9%           
======================================
  Files          95      95           
  Lines        9051    9051           
  Branches     1011    1011           
======================================
  Hits         9042    9042           
  Misses          5       5           
  Partials        4       4

Impacted Files	Coverage Δ
faust/streams.py	100% <100%> (ø)	⬆️

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