fix: skip empty post upgrade task

[oliverpool]

Changes

Skip empty postUpgradeTasks.commands (after compilation).

Explanation

When using templates, the resulting postUpgradeTasks.commands can become empty.
In my case, I want to run the command only for a specific manager:

  "postUpgradeTasks": {
    "commands": [
      "{{#if (equals manager 'jsonata')}}lon modify {{{depName}}} --revision {{{ newDigest }}}{{/if}}"
    ],
    "fileFilters": [
      "lon.lock"
    ],
    "executionMode": "update"
  }

This currently fails for other managers (like cargo), with a message Post-upgrade command '' has not been added to the allowed list in allowedCommands.

Current workaround: use an {{else}}true condition and add ^true$ to the allowedCommands (skipping the command should be faster and easier to manage).

Context

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