Fix git tag generation

[dappnodedev]

SDK publish action in mono-variant package repos would fail with this error:

[07:14:41] Release on github [started]
[07:14:41] Handle tags [started]
[07:14:41] Handle tags [failed]
[07:14:41] → Missing required release details for dappmanager.dnp.dappnode.eth
[07:14:41] Release on github [failed]
[07:14:41] → Missing required release details for dappmanager.dnp.dappnode.eth
 ✖ Missing required release details for dappmanager.dnp.dappnode.eth

Error: Missing required release details for dappmanager.dnp.dappnode.eth
    at buildReleaseDetailsMap (file:///home/runner/.npm/_npx/7366de4368e80aba/node_modules/@dappnode/dappnodesdk/src/tasks/createGithubRelease/buildReleaseDetailsMap.ts:14:13)
    at Task.task (file:///home/runner/.npm/_npx/7366de4368e80aba/node_modules/@dappnode/dappnodesdk/src/tasks/createGithubRelease/subtasks/getHandleTagsTask.ts:30:33)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)

This PR aims to fix this bug by handling the case where variant is null. This happens when there are no variants for a package repo