fix(retainer): fix that EMQX can't start when the retainer is disabled

[lafirest]

Reproduce steps:

1. Start an emqx and disable the Retainer
2. Restarting emqx will report the following error

[codeball-ai-writer]

Codeball: LGTM! 👍

confidence: 0.98 | [dashboard] | [adjust AI settings]

[coveralls]

Pull Request Test Coverage Report for Build 3014601779

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 78 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.09%) to 77.323%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.31%
apps/emqx_connector/src/emqx_connector_http.erl	1	71.19%
apps/emqx_machine/src/emqx_machine.erl	1	79.31%
apps/emqx_resource/src/emqx_resource_manager.erl	1	87.25%
apps/emqx/src/emqx_broker.erl	1	81.87%
apps/emqx/src/emqx_stats.erl	1	93.33%
apps/emqx/src/emqx_ws_connection.erl	1	79.01%
apps/emqx/src/emqx_alarm.erl	2	91.74%
apps/emqx/src/emqx_channel.erl	2	86.36%
apps/emqx_management/src/emqx_mgmt_cli.erl	5	29.37%

Totals
Change from base Build 3005506636:	-0.09%
Covered Lines:	20605
Relevant Lines:	26648

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💛 - Coveralls