tolerate null sources on projected volumes

[zapman449]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Removes false requirement that projected volume sources be a populated list (rather than null)

Which issue(s) this PR fixes:

Fixes #1494

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NONE

Removed restriction on Projected Volume sources being `null`/`None`

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[zapman449]

/check-cla

[roycaihw]

/lgtm
/approve

After this PR is merged, could you also add a code section to apply-hotfixes.sh, similar to the code here? Basically replace the commit hash with the commit hash of this PR, and put reasonable log message. The apply-hotfixes.sh is used as part of our release process, it will help prevent the code change in this PR being wiped out by the code generator, when we generate a new client using the OpenAPI spec from a Kubernetes version < 1.20.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: roycaihw, zapman449

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