【FIX】fix revisionHistoryLimit is not exist

[JameKeal]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When I deploy yurt-app-manager, and the cluster is also have some uniteddeployment, the yurt-app-manager is not run success, the error is:

The reason is ud.Spec.RevisionHistoryLimit is not exist, but the code use it's pointer to get the value.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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In response to this:

What type of PR is this?

/kind bug

What this PR does / why we need it:

When I deploy yurt-app-manager, and the cluster is also have some uniteddeployment, the yurt-app-manager is not run success, the error is:

The reason is ud.Spec.RevisionHistoryLimit is not exist, but the code use it's pointer to get the value.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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[rambohe-ch]

/lgtm
/approve

[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JameKeal, rambohe-ch

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