fix: provider crash when retrieving apps with datasource_apps

[ANUGRAHG]

Purpose

• This PR fixes a runtime crash that occurs when using the data.cloudfoundry_apps data source in configuration where at least one application was pushed from a Docker image with --docker-username.
• closes [BUG] Crash when retrieving docker apps with datasource_apps #340

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 ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

Other Information

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• 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.