dm: fix empty config cause dm-master panic (#5298)

[ti-chi-bot]

This is an automated cherry-pick of #5298

What problem does this PR solve?

Issue Number: close #3732

What is changed and how it works?

check rule is empty. If so, the ErrConfigGlobalConfigsUnused will be returned

Tests

• Integration test

Release note

fix the issue that empty dump/load/sync config will cause dm-master panic.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
jenkins-ticdc/verify	b1ef632	link	true	/test verify
pull-dm-integration-test	b1ef632	link	true	/test dm-integration-test
pull-dm-compatibility-test	b1ef632	link	true	/test dm-compatibility-test

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