fix(ec2): ignore all-zero /sys/hypervisor/uuid

[Sanjays2402]

Proposed Commit Message

fix(ec2): ignore all-zero /sys/hypervisor/uuid

On a Xen dom0 /sys/hypervisor/uuid reads all zeros.
_collect_platform_data() prefers that file over DMI and an all-zero
uuid is truthy, so the DMI fallback never ran and AWS was not
identified. No IMDSv2 token was then requested, and with
HttpTokens=required the tokenless IMDSv1 requests failed for the full
240 second wait before the datasource gave up.

Treat the null uuid as absent so the DMI system-uuid is used.

Fixes GH-6937

Additional Context

Fixes #6937

Reported in #6937 with logs from an Ubuntu 24.04 EC2 instance booted as a Xen dom0.

Test Steps

tests/unittests/sources/test_ec2.py::TestIdentifyPlatform::test_identify_aws_null_hypervisor_uuid mocks the null /sys/hypervisor/uuid and an ec2... DMI system-uuid; it fails on the current code (unknown) and passes with this change (aws). The rest of tests/unittests/sources/test_ec2.py is unchanged and green (67 passed).

On a live system: boot an EC2 instance with --metadata-options HttpTokens=required as a Xen dom0, then check that cloud-init identifies the AWS datasource instead of waiting out the 240 second IMDSv1 timeout and falling back to DataSourceNone.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)