fix(datasource/helm): Digests are blank on Gitlab

[mpkorstanje]

Changes

Ensures that newDigest is not set to null when it is left blank in the index.yaml.

Context

In #23432 support was added for digests. This introduces a regression when used in combination with the Helm repository on Gitlab. As a result packages hosted in the Gitlab Helm repository are no longer updated.

In the index.yaml generated by Gitlabs Helm Repository the digest is left blank. For example:

apiVersion: v2
entries:
  blank-digest:
  - apiVersion: v2
    created: '2023-09-05T13:25:19.046604000Z'
    description: A Helm chart with a blank digest
    digest:
    name: blank-digest
    urls:
    - https://charts.helm.sh/stable/packages/ambassador-2.7.0.tgz
    version: 2.7.0

As a result newDigest: release.digest will set the newDigest to null

renovate/lib/modules/datasource/helm/index.ts

Lines 68 to 75 in 2431c5a

	result[name] = {
	homepage: latestRelease.home,
	sourceUrl,
	releases: releases.map((release) => ({
	version: release.version,
	releaseTimestamp: release.created ?? null,
	newDigest: release.digest,
	})),

and this causes the update to be removed in workers/repository/process/lookup/index.ts.

renovate/lib/workers/repository/process/lookup/index.ts

Lines 464 to 469 in 2431c5a

	// Strip out any non-changed ones
	res.updates = res.updates
	.filter(
	(update) => update.newValue !== null || config.currentValue === null
	)
	.filter((update) => update.newDigest !== null)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

otherwise LGTM

[renovate-release]

🎉 This PR is included in version 36.87.7 🎉

The release is available on:

• GitHub release
• 36.87.7

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