fix(paging): respect matchspec even if qs is empty when counting

[keynslug]

Before this fix, to provide count in the response metadata we might end up counting all the elements in the underlying table even if the query involves a match specification.

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EMQX-8720

[coveralls]

Pull Request Test Coverage Report for Build 3910877797

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• 36 unchanged lines in 17 files lost coverage.
• Overall coverage decreased (-0.01%) to 79.982%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_management/src/emqx_mgmt_api.erl	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.86%
apps/emqx_machine/src/emqx_machine.erl	1	79.31%
apps/emqx_management/src/emqx_mgmt_api_trace.erl	1	86.49%
apps/emqx/src/emqx_broker.erl	1	84.07%
apps/emqx/src/emqx_flapping.erl	1	85.11%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_os_mon.erl	1	84.06%
apps/emqx/src/emqx_router_helper.erl	1	86.27%
apps/emqx/src/emqx_schema.erl	1	90.91%
apps/emqx/src/emqx_session.erl	1	87.2%

Totals
Change from base Build 3910231074:	-0.01%
Covered Lines:	23010
Relevant Lines:	28769

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💛 - Coveralls