Add nil check for connection ack

[vkuznecovas]

Fixes #1943

[codecov-io]

Codecov Report

Merging #1947 into master will increase coverage by 0.11%.
The diff coverage is 61.18%.

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
+ Coverage   47.98%   48.10%   +0.11%     
==========================================
  Files         287      289       +2     
  Lines       12617    12640      +23     
==========================================
+ Hits         6054     6080      +26     
+ Misses       5998     5995       -3     
  Partials      565      565              

Impacted Files	Coverage Δ
core/service/subscription.go	0.00% <0.00%> (ø)
session/pingpong/exchange_messaging.go	0.00% <0.00%> (ø)
session/pingpong/factory.go	0.00% <0.00%> (ø)
session/pingpong/invoice_messaging.go	0.00% <0.00%> (ø)
core/service/manager.go	58.53% <50.00%> (ø)
p2p/listener.go	60.44% <60.44%> (ø)
p2p/dialer.go	61.22% <61.22%> (ø)
p2p/common.go	64.91% <64.91%> (ø)
p2p/channel.go	76.19% <76.92%> (-1.37%)	⬇️
core/connection/manager.go	71.95% <80.00%> (+0.22%)	⬆️
... and 5 more

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