getInput returns undefined when an empty string is used as input for optional task input

[ryenus]

We have an optional task input parameter named dockerImage in our custom azure task:

    {
      "name": "dockerImage",
      "type": "string",
      "label": "Docker Image",
      "defaultValue": "",
      "required": false,
      "helpMarkDown": "The Docker image to run the step in.",
      "groupName": "docker"
    }

When the input parameter is omitted, getInput returns undefined as we'd expected, however, when the user explicitly set dockerImage to an empty string using dockerImage: "", we also got undefined instead of an zero length string as we would expect.

We'd expect a fix so that we could:

1. activate a fallback logic when the input is omitted / undefined
2. skip docker container creation if the input is an empty string / ''

Currently we use azure-pipelines-task-lib 4.4.0 but the issue likely has existed for years.