New machine should not move to the pending state if node label is missing

[aaronfern]

What this PR does / why we need it:
We recently ran into a bug where a machine was stuck in the Pending state while its corresponding node had already joined the cluster.
The reason for this was that the node name label on the machine had not yet been populated.
This can occur due to the an oversight in the machine creation flow where a newly created machine is moved to the Pending state after it's underlying node (or VM) has been created and initialised. The machine creation flow makes a best effort attempt to populate they node label, but it does not requeue the machine if it could not.
The reason we do not want machines to move to the Pending state in this case is that when a machine is in this state, it is not allowed to reenter the machine creation flow since we are just waiting for the node to join the k8s cluster. This causes an issue as the node name label is only updated by the machine creation flow and the machine deletion flow, leading to machines being stuck in this state indefinitely

These kind of bugs can surface when a CSP is not able to provide a node name on time. The machine creation flow in turn moved the machine to the Pending state after initialising the node (VM) and this machine is stuck

This PR enhances the machine creation flow to not allow newly created machines to proceed to the Pending phase when the NodeName and ProviderID are not available.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

newly created machines not moved to `Pending` state without valid node name and providerID from provider

[r4mek]

Other than that code changes seem fine. Should handle the case.

[takoverflow]

Thanks for the fix, LGTM!

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[gardener-prow]

LGTM label has been added.

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Git tree hash: aa1ba4d084c5a3462a5b56027cd4cb0951f4fdbf

[takoverflow]

/cherry-pick rel-v0.61

[gardener-ci-robot]

@takoverflow: once the present PR merges, I will cherry-pick it on top of rel-v0.61 in a new PR and assign it to you.

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/cherry-pick rel-v0.61

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[gardener-ci-robot]

@takoverflow: new pull request created: #1072

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/cherry-pick rel-v0.61

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