fix interpretation of time.Time{} as a pacing deadline

[marten-seemann]

Fixes #2974. Closes #2977 (that PR just fixes the symptoms, not the underlying problem).

The pacer returns the zero value of time.Time when a packet can be sent immediately. The session uses the zero value to unset the pacing deadline.

[codecov]

Codecov Report

Merging #2980 (33d058f) into master (4653549) will increase coverage by 0.76%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2980      +/-   ##
==========================================
+ Coverage   86.12%   86.88%   +0.76%     
==========================================
  Files         135      135              
  Lines        9379    10029     +650     
==========================================
+ Hits         8077     8713     +636     
- Misses        943      947       +4     
- Partials      359      369      +10     

Impacted Files	Coverage Δ
internal/congestion/pacer.go	100.00% <ø> (ø)
session.go	82.25% <100.00%> (+4.39%)	⬆️
zero_rtt_queue.go	92.31% <0.00%> (-1.81%)	⬇️
internal/ackhandler/received_packet_handler.go	75.00% <0.00%> (-1.12%)	⬇️
qlog/event.go	98.26% <0.00%> (+0.51%)	⬆️
qlog/qlog.go	95.59% <0.00%> (+0.97%)	⬆️
qlog/packet_header.go	96.92% <0.00%> (+1.27%)	⬆️
internal/wire/transport_parameters.go	91.02% <0.00%> (+2.40%)	⬆️
logging/multiplex.go	100.00% <0.00%> (+2.56%)	⬆️
server.go	86.35% <0.00%> (+4.00%)	⬆️
... and 1 more

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