fix(datasource/docker): handle empty newValue in getDigest

[viceice]

Changes

newValue can be an empty string, so ?? operator doesn't fallback to latest tag.
This causes wrong docker api requests.

DEBUG: getDigest(https://code.forgejo.org/, forgejo/forgejo, ) (repository=forgejo-helm/forgejo-helm, baseBranch=main)
DEBUG: getManifestResponse(https://code.forgejo.org/, forgejo/forgejo, , head) (repository=forgejo-helm/forgejo-helm, baseBranch=main)
...
DEBUG: HEAD https://code.forgejo.org/v2/forgejo/forgejo/manifests/ = (code=ERR_NON_2XX_3XX_RESPONSE, statusCode=404 retryCount=0, duration=17) (repository=forgejo-helm/forgejo-helm, baseBranch=main)
DEBUG: Docker Manifest is unknown (repository=forgejo-helm/forgejo-helm, baseBranch=main)
       "err": {
         "name": "HTTPError",
         "code": "ERR_NON_2XX_3XX_RESPONSE",
         "timings": {
           "start": 1736256846663,
           "socket": 1736256846663,
           "lookup": 1736256846663,
           "connect": 1736256846663,
           "secureConnect": 1736256846663,
           "upload": 1736256846663,
           "response": 1736256846680,
           "end": 1736256846680,
           "phases": {
             "wait": 0,
             "dns": 0,
             "tcp": 0,
             "tls": 0,
             "request": 0,
             "firstByte": 17,
             "download": 0,
             "total": 17
           }
         },
         "message": "Response code 404 (Not Found)",
...
DEBUG: Could not determine new digest for update. (repository=forgejo-helm/forgejo-helm, baseBranch=main)
       "packageName": "code.forgejo.org/forgejo/forgejo",
       "currentValue": "",
       "datasource": "docker",
       "newValue": "",
       "bucket": undefined

Context

Documentation (please check one with an [x])

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• 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

[renovate-release]

🎉 This PR is included in version 39.92.1 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.92.1

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