fix(graphql): filter out orphaned relationships

[admosity]

Description of what you did:

We have entities that reference another entity that could be deleted. For performance reasons, any entity that is orphaned is cleaned at a later time, but this is breaking our graphql aggregation queries. This PR adds a simple existence filter on the _id field that exists when null as well. Maybe it might also be beneficial to allow grouping by null to check orphaned relationships?

Output from a log above converting the result to an array:

Output from the graphql query:

{
  "errors": [
    {
      "message": "Cannot read property 'toString' of null",
      "locations": [
        {
          "line": 4,
          "column": 7
        }
      ],
      "path": [
        "entriesConnection",
        "groupBy",
        "release"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "TypeError: Cannot read property 'toString' of null",
            "    at /Users/me/git/proj/redacted/node_modules/strapi-plugin-graphql/services/build-aggregation.js:220:22",
            "    at arrayMap (/Users/me/git/proj/redacted/node_modules/strapi-plugin-graphql/node_modules/lodash/lodash.js:639:23)",
            "    at Function.map (/Users/me/git/proj/redacted/node_modules/strapi-plugin-graphql/node_modules/lodash/lodash.js:9554:14)",
            "    at preProcessGroupByData (/Users/me/git/proj/redacted/node_modules/strapi-plugin-graphql/services/build-aggregation.js:217:12)",
            "    at resolver (/Users/me/git/proj/redacted/node_modules/strapi-plugin-graphql/services/build-aggregation.js:274:14)",
            "    at processTicksAndRejections (internal/process/task_queues.js:93:5)"
          ]
        }
      }
    }
  ],
  "data": {
    "entriesConnection": {
      "groupBy": {
        "release": null
      }
    }
  }
}

[codecov]

Codecov Report

Merging #7363 into master will increase coverage by 0.05%.
The diff coverage is 48.64%.

@@            Coverage Diff             @@
##           master    #7363      +/-   ##
==========================================
+ Coverage   26.17%   26.22%   +0.05%     
==========================================
  Files        1130     1131       +1     
  Lines       15415    15433      +18     
  Branches     2439     2443       +4     
==========================================
+ Hits         4035     4048      +13     
- Misses       9561     9565       +4     
- Partials     1819     1820       +1     

Flag	Coverage Δ
#front	18.26% <0.00%> (-0.01%)	⬇️
#unit	53.14% <100.00%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...s/strapi-admin/admin/src/containers/Admin/index.js	63.82% <ø> (ø)
...admin/admin/src/containers/Roles/EditPage/index.js	2.77% <0.00%> (-0.80%)	⬇️
...admin/admin/src/containers/Roles/ListPage/index.js	0.00% <0.00%> (ø)
...ermissions/admin/src/components/PopUpForm/index.js	0.00% <0.00%> (ø)
packages/strapi-admin/services/metrics.js	100.00% <100.00%> (ø)
packages/strapi-admin/services/role.js	94.11% <100.00%> (+0.14%)	⬆️
packages/strapi-admin/services/user.js	82.56% <100.00%> (+0.66%)	⬆️
packages/strapi/lib/services/metrics/sender.js	100.00% <100.00%> (ø)

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[alexandrebodin]

LGTM, thanks !