fix: clean null values out of arrays

[clareliguori]

Fixes #53

The following task definition input for the action:

      "logConfiguration": {
        "logDriver": "awsfirelens",
        "options": {
          "KeyName": ""
        },
        "secretOptions": [
          {
            "name": "",
            "valueFrom": ""
          }
        ]
      },

currently generates the following input to RegisterTaskDefinition:

      "logConfiguration": {
        "logDriver": "awsfirelens",
        "secretOptions": [
          null
        ]
      },

Because of the null value, the only error message is Cannot read property 'name' of null. This PR removes all the null values so that a proper error message is given to the user.

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[piradeepk]

Ship it! 💯

[hossain-rayhan]

looks good.