Mutate empty Tier set in Antrea-native policies to default tier

[abhiraut]

Fixes: #1550

[antrea-bot]

Thanks for your PR.
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[codecov-io]

Codecov Report

Merging #1567 (467de2b) into master (9d3d10b) will decrease coverage by 1.03%.
The diff coverage is 46.92%.

@@            Coverage Diff             @@
##           master    #1567      +/-   ##
==========================================
- Coverage   63.31%   62.28%   -1.04%     
==========================================
  Files         170      181      +11     
  Lines       14250    15160     +910     
==========================================
+ Hits         9023     9442     +419     
- Misses       4292     4702     +410     
- Partials      935     1016      +81     

Flag	Coverage Δ
kind-e2e-tests	52.70% <44.93%> (-2.68%)	⬇️
unit-tests	40.66% <10.49%> (-0.61%)	⬇️

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

Impacted Files	Coverage Δ
cmd/antrea-agent/agent.go	0.00% <0.00%> (ø)
cmd/antrea-agent/options.go	21.69% <0.00%> (+0.97%)	⬆️
pkg/agent/config/node_config.go	100.00% <ø> (ø)
.../agent/flowexporter/connections/conntrack_linux.go	28.37% <0.00%> (-0.79%)	⬇️
pkg/agent/stats/collector.go	97.72% <ø> (ø)
pkg/controller/networkpolicy/tier.go	90.00% <ø> (ø)
pkg/features/antrea_features.go	16.66% <ø> (ø)
pkg/ovs/openflow/ofctrl_builder.go	60.58% <0.00%> (-1.59%)	⬇️
pkg/ovs/openflow/ofctrl_group.go	48.57% <0.00%> (-4.56%)	⬇️
pkg/ovs/openflow/ofctrl_action.go	51.26% <5.71%> (-10.10%)	⬇️
... and 45 more

[tnqn]

An inconsistent comment, otherwise LGTM

[abhiraut]

/test-all

[abhiraut]

Merging as the latest commit only addresses a typo in comments and have 2 lgtm