Infrastructure: Fix machine placement target defaults

[cjschaef]

Make sure that the optional fields for the
VPCMachinePlacementTarget definition are configured properly for JSON serialization.

Related: #2150

What this PR does / why we need it:
Fixes bug with Infrastructure Machines attempting to use Dedicated Host support

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2150

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Fix bug with placementTarget.dedicatedHostGroup field being omitted when no provided

[k8s-ci-robot]

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[mkumatag]

/ok-to-test

[Prajyot-Parab]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cjschaef, Prajyot-Parab

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