Allow null and empty string values for unused field

What does this MR do and why?

Allow null and empty string values for unused field

API URL field on Prometheus integrations is no longer used. It should not be required via API.

Fixes specs that test this functionality.

Changelog: changed

Screenshots or screen recordings

NA

How to set up and validate locally

1. Use the following GQL mutation to create an entry for Integration -

mutation PrometheusIntegrationCreate($input: PrometheusIntegrationCreateInput!) {
  prometheusIntegrationCreate(input: $input) {
    integration {
      id
      name
      active
      apiUrl
      url
    }
    errors
  }
}

with following same input -

{
  "input": {
    "projectPath": "<project_path>",
    "active": true
  }
}

2. The above mutation should return success and integration should be created.

MR acceptance checklist

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