Fix channel crash for slow clients with enhanced authentication

[savonarola]

Also gracefully handle Authentication-Data absence.

[coveralls]

Pull Request Test Coverage Report for Build 4376043824

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• 41 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.04%) to 80.829%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_authn/src/enhanced_authn/emqx_enhanced_authn_scram_mnesia.erl	1	88.06%
apps/emqx_bridge/src/emqx_bridge.erl	1	89.6%
apps/emqx_gateway/src/mqttsn/emqx_sn_channel.erl	1	73.13%
apps/emqx_machine/src/emqx_machine.erl	1	77.42%
apps/emqx_resource/src/emqx_resource_manager.erl	1	92.04%
apps/emqx/src/emqx_broker.erl	1	85.96%
apps/emqx/src/emqx_channel.erl	1	87.6%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_session.erl	1	87.6%
apps/emqx/src/emqx_stats.erl	1	91.67%

Totals
Change from base Build 4362708326:	-0.04%
Covered Lines:	24235
Relevant Lines:	29983

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💛 - Coveralls

[savonarola]

Coverage decreased although all new lines are covered 🤷