backup: fix gke wif with empty credential file (#6888)

[ti-chi-bot]

This is an automated cherry-pick of #6888

[ti-chi-bot]

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[Leavrth]

/retest

[ti-chi-bot]

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

Test name	Commit	Details	Required	Rerun command
pull-e2e-kind-tidbcluster	5f26bb9	link	false	/test pull-e2e-kind-tidbcluster

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