Add default value to an empty array using JOLT

I have this JSON as input:

{
  "my_array": []
}

If the array is empty I need to add a placeholder to the array, so this is the desired output:

{
  "my_array": ["placeholder"]
}

If the array has a value/s:

{
  "my_array": ["I have a value", "I am a second value"]
}

I need to keep those values on the array, so the desired output should be the equal to the input:

{
  "my_array": ["I have a value", "I am a second value"]
}

So far I have tried using the default operation without success:

[
  {
    "operation": "default",
    "spec": {
      "my_array[]": [
        "placeholder"
      ]
    }
  }
]

And I have also tried using a conditional with the shift operation without success:

[
  {
    "operation": "shift",
    "spec": {
      "my_array": {
        "[]": {
          "#placeholder": "my_array"
        },
        "*": {
          "@(2,my_array)": "my_array"
        }
      }
    }
  }
]

This last code throws the error:

> Error running the Transform. JOLT Chainr encountered an exception
> constructing Transform className:com.bazaarvoice.jolt.Shiftr at index:0.

You can handle the issue through measuring the size of the array and put into the conditional whether it's equal to zero or not such as

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "sz_array": "=size(@(1,my_array))"
    }
 },
 {
    "operation": "shift",
    "spec": {
      "sz_array": {
        "0": {
          "@2,my_array": { "#placeholder": "my_array[]" }
        },
        "*": {
          "@2,my_array": "my_array"
        }
      }
    }
  }
]

Edit : Considering that you have the older versions of the Jolt library, you can prefer using the following spec as an alternative

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "xyz": ["=toInteger(@(1,my_array))", ["0"]]
    }
  },
  {
    "operation": "shift",
    "spec": {
      "xyz": {
        "*": {
          "0": {
            "@3,my_array": { "#placeholder": "my_array[]" }
          },
          "*": {
            "@3,my_array": "my_array"
          }
        }
      }
    }
  }
]

The first answer provided Barbaros Özhan works correctly. But if you have a jolt library version that does not support the size function, this solution should work as well:

[
  {
    "operation": "modify-overwrite-beta",
    "spec": {
      "my_array_joined": "=join('',@(1,my_array))"
    }
  },
  {
    "operation": "shift",
    "spec": {
      "my_array_joined": {
        "": {
          "#placeholder": "my_array"
        },
        "*": {
          "@(2,my_array)": "my_array"
        }
      }
    }
  }
]

Edit: The second solution provided by Barbaros Özhan works as well