fix: Bug in HTML report generation where feed metadata can be null

[bdferris-v2]

This fixes a bug introduced in #1363 when metadata was added to the validation HTML report. The metadata data structure can be null when a system error occurs when processing a feed, which ultimately results in a NPE when generating the HTML report.

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[briandonahue]

LGTM

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1428 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1428 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1428 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1428 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1428 sources (~0 %) are corrupted.
Commit: c3a0629
Download the full acceptance test report here (report will disappear after 90 days).
✅ Rule acceptance tests passed.