Fix API Server panic when volume component has no `ephemeral` field set

[rm3l]

What type of PR is this: /kind bug

What does this PR do / why we need it:

Which issue(s) this PR fixes: Fixes #7079

PR acceptance criteria:

• [x] Unit test

• [ ] Integration test

• [ ] Documentation

How to test changes / Special notes to the reviewer: See the retro steps in #7079

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