Fix crash while updating app with rolling strategy

[Dray56]

Purpose

• fix [BUG] updating a cloudfoundry_app routinely crashes the provider #101

Does this introduce a breaking change?

• Yes
• No

Pull Request Type

What kind of change does this Pull Request introduce?

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Other... Please describe:

How to Test

• Test the code via automated test

go test ./...

• Additional test steps

...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

• ...

Other Information

Checklist for reviewer

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• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.