backup: fix gke wif with empty credential file

[Leavrth]

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[liubog2008]

Please change base to release-1.x. Thanks

[ti-chi-bot]

@Leavrth: 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
pull-e2e-kind-dmcluster	e14ab3e	link	false	/test pull-e2e-kind-dmcluster
pull-e2e-kind-basic	e14ab3e	link	false	/test pull-e2e-kind-basic
pull-e2e-kind-scale-simultaneously	e14ab3e	link	false	/test pull-e2e-kind-scale-simultaneously
pull-e2e-kind-serial	e14ab3e	link	false	/test pull-e2e-kind-serial
pull-e2e-kind-tngm	e14ab3e	link	false	/test pull-e2e-kind-tngm
pull-e2e-kind-tidbcluster	e14ab3e	link	false	/test pull-e2e-kind-tidbcluster
pull-e2e-kind-across-kubernetes	e14ab3e	link	false	/test pull-e2e-kind-across-kubernetes
pull-e2e-kind-br	e14ab3e	link	false	/test pull-e2e-kind-br

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[Leavrth]

closed by #6888