make network optional and empty if hub is present

[amanMahendroo]

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networkconnectivity: made `Transport.network` optional and passed empty string if `Transport.hub` is present (b/509697933)

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@slevenick, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit b7659bd:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	2 files changed, 20 insertions(+), 13 deletions(-)
google-beta provider	View Diff	2 files changed, 20 insertions(+), 13 deletions(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
41	40	1	0

Affected Service Packages

• networkconnectivity

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[slevenick]

Please do a local review with a Google engineer from the service team as described at go/terraform-service-review. Let me know when that's finished and I'll follow up with final review & merge.

[harshithpatte-g]

Can't we use https://googlecloudplatform.github.io/magic-modules/reference/field/#conflicts?

[amanMahendroo]

@harshithpatte-g b/509697933 seems to have specific instructions to send empty string when a hub is present. Using conflicting fields would send null value instead and disallow users to populate a network field. I believe we want to allow users to put in both hub and network fields but in this case network would be overridden by an empty string.

[harshithpatte-g]

@harshithpatte-g b/509697933 seems to have specific instructions to send empty string when a hub is present. Using conflicting fields would send null value instead and disallow users to populate a network field. I believe we want to allow users to put in both hub and network fields but in this case network would be overridden by an empty string.

In that case, the customer can set any random value to be network field.
I have added a comment on the bug.