Issue Type: Bug

Verifying #50819

I can replace the top-level shell args with some other shell args on the task. Except: I cannot replace them with the empty list.

This task still prints the shell's version although its shell.args are empty, I'd expect it to print 'Hello':

  "options": {
    "shell": {
        "args": [
            "--version"
        ]
    }
  },
  "tasks": [
    {
      "type": "shell",
      "label": "echo",
      "command": "echo Hello",
      "options": {
        "shell": {
            "args": [
            ]
        }
      },
    }
  ]

VS Code version: Code - Insiders 1.25.0-insider (4991227, 2018-06-29T05:11:09.904Z)
OS version: Darwin x64 17.6.0