fix: mqtt bridge payload default value

[JimMoen]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 3044482484

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 29 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.01%) to 77.481%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx/src/bpapi/emqx_bpapi.erl	1	95.83%
apps/emqx/src/emqx_schema.erl	1	90.91%
apps/emqx/src/emqx_stats.erl	1	93.33%
apps/emqx/src/emqx_config_handler.erl	2	96.21%
apps/emqx_gateway/src/emqx_gateway_cm.erl	3	73.45%
apps/emqx_gateway/src/exproto/emqx_exproto_gcli.erl	3	48.78%
apps/emqx_management/src/emqx_mgmt_cli.erl	5	29.37%
apps/emqx/src/emqx_sys_mon.erl	6	85.42%
apps/emqx/src/emqx_reason_codes.erl	7	93.38%

Totals
Change from base Build 3044431786:	-0.01%
Covered Lines:	20713
Relevant Lines:	26733

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💛 - Coveralls