fix: send an empty array for IP ranges instead of null

[ctreatma]

Fix #515.

The TestAccMetalVRF_withIPRanges started failing recently with an error that ip_ranges cannot be null. It appears this test failure was caused by #509, which consolidated the expandListToStringList and IfArrToStringArr functions.

This updates the IfArrToStringArr converter to match the old expandListToStringList implementation, which fixes the failing test.

[ctreatma]

FWIW, I don't really get why this implementation change is needed.

Near as I can tell, this was the line that was leading to test failure: https://github.com/equinix/terraform-provider-equinix/blob/main/equinix/resource_metal_vrf.go#L107

...but it seems like &ipRanges should have been non-nil in both cases based on this: https://go.dev/play/p/8vXQ6PWFi0Q

What am I missing?

More comprehensive Go sample courtesy of @displague: https://go.dev/play/p/aEANmMgMV0A

In most situations a nil slice and an empty slice are functionally equivalent, but JSON serialization is one situation where they are not functionally equivalent.

[ctreatma]

Test passing again in CI: https://github.com/equinix/terraform-provider-equinix/actions/runs/7493501381/job/20399350107#step:5:500

[codecov-commenter]

Codecov Report

Attention: 700 lines in your changes are missing coverage. Please review.

Comparison is base (26b2f3d) 60.11% compared to head (b9136db) 59.11%.
Report is 128 commits behind head on main.

Files	Patch %	Lines
internal/errors/errors.go	7.58%	134 Missing ⚠️
equinix/fabric_mapping_helper.go	0.00%	69 Missing ⚠️
equinix/resource_fabric_connection.go	0.00%	53 Missing ⚠️
equinix/resource_fabric_cloud_router.go	0.00%	43 Missing ⚠️
equinix/resource_network_device.go	2.50%	39 Missing ⚠️
equinix/resource_fabric_service_profile.go	0.00%	26 Missing ⚠️
internal/resources/metal/metal_ssh_key/resource.go	73.62%	19 Missing and 5 partials ⚠️
equinix/resource_fabric_routing_protocol.go	0.00%	23 Missing ⚠️
equinix/resource_metal_device.go	74.71%	18 Missing and 4 partials ⚠️
equinix/resource_metal_virtual_circuit.go	50.00%	13 Missing and 2 partials ⚠️
... and 43 more

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
- Coverage   60.11%   59.11%   -1.01%     
==========================================
  Files          99       96       -3     
  Lines       20045    19842     -203     
==========================================
- Hits        12051    11729     -322     
- Misses       7691     7822     +131     
+ Partials      303      291      -12     

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[ctreatma]

The IfArrToStringArr function was introduced in #473; I looked through that PR and (1) the only things that were using that function prior to #509 were Metal resources and data sources (which means they're fairly well covered by tests) and (2) where it was used, it looks like it should be OK to return [] in the cases where that would have returned []-but-actually-nil.

[ocobles]

lgtm. I didn't try it but I tried the playground example