fix: Prevent GCPManagedCluster reconciler to not crash if Network.Name and Network.Subnets is not passed to it

[tasdikrahman]

What type of PR is this?

/kind bug

What this PR does / why we need it:

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 #1187

Special notes for your reviewer:

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

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

fix: Prevent GCPManagedCluster reconciler to not crash if Network.Name and Network.Subnets is not passed to it

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @tasdikrahman. Thanks for your PR.

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[damdo]

Thanks a lot @tasdikrahman this looks good!
Would you be able to add also some unit tests to exercise that?
Thanks!

[cpanato]

/ok-to-test

[cpanato]

/hold for the unit tests

thanks for working on this !

[tasdikrahman]

Hello folks, I re-opened this PR with the fixes and test specifications here #1284, please have a look. @cpanato @damdo