[Validators] Fix error message returned by validator ExistingFsxNetworkingValidator when Fsx has no ENI

[gmarciani]

Description of changes

Fixed issue with validator ExistingFsxNetworkingValidator so that, when a shared storage does not return network interfaces, the validator fails with a clear error message instead of a generic NoneType error.

UX

To validate the scenario I am triggering the pcluster update-cluster to mount an external FSx that does not have the ENI attached yet (it is still creating).

Before the fix

  "configurationValidationErrors": [
    {
      "level": "ERROR",
      "type": "ExistingFsxNetworkingValidator",
      "message": "'NoneType' object is not iterable"
    },

After the fix

  "configurationValidationErrors": [
    {
      "level": "ERROR",
      "type": "ExistingFsxNetworkingValidator",
      "message": "Unable to validate FSx security groups. The given FSx file storage 'fs-09ae44abbca688213' doesn't have Elastic Network Interfaces attached to it."
    }
  ],

Tests

1. Manually tested (see UX above)

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