[BUG] MapToList processor throws NPE when input map contains null values

[oeyh]

Describe the bug
MapToList processor throws NPE when input map contains null values

To Reproduce
For example, if the input data is:

  {
    "my-map": {
      "key1": "value1",
      "key2": null
    }
  }

and processor config is:

    - map_to_list:
        source: "my-map"
        target: "my-list"

or

    - map_to_list:
        source: "my-map"
        target: "my-list"
        convert_field_to_list: true

It will throw NPE.

Expected behavior
Processor should be able to process the record without errors.

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Ubuntu 20.04 LTS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.