Fix empty attachments 500 error in NPM package PUT request

What does this MR do and why?

When the API path PUT /api/:version/projects/:id/packages/npm/:package_name is called and the body of the request contains an attachment with a data field that is an empty string the API returns a 500 error with the message undefined method count' for nil:NilClass`.

Solution

Catch the error and raise 400 error instead of 500 with a message explaining the reason for the error to the user: {"message":"Attachment data is empty."}

Screenshots or screen recordings

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How to set up and validate locally

1. Have a project where the packages feature is enabled
2. Create a payload.json file with the following payload

payload

{
    "_attachments": {
        "foobar-1.0.0.tgz": {
            "data": ""
        }
    },
    "_id": "foobar@1.0.0",
    "name": "foobar",
    "dist-tags": {
        "latest": "1.0.2"
    },
    "versions": {
        "1.0.0": {
            "name": "foobar",
            "version": "1.0.0",
            "dist": {
                "shasum": "675ea51efc43db9e74c8c6b354adc2f5db1d0221"
            },
            "tarball": "https://example.com/foobar.tgz",
            "_id": "foobar@1.0.0"
        }
    }
}

3. Run this curl command (with a valid token and project ID):

curl --request PUT --header "Content-Type: application/json" --data @./payload.json --header "Authorization: Bearer your-token-here" http://gdk.test:3000/api/v4/projects/:project_id_here/packages/npm/foobar

4. You should receive a 400 error with {"message":"Attachment data is empty."}. On master branch you would get 500 error.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #403162 (closed)