Fix input value coercion with defaults (#1073)

[ilslv]

Resolves #1073

Solution

Don't additionally wrap field's type into an Option and don't replace missing Variables with InputValue::Null too soon.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains Draft: prefix
  • [x] Name contains issue reference
  • [x] Has assignee
• [x] Documentation is updated (if required)
• [x] Tests are updated (if required)
• [x] Changes conform code style
• [x] CHANGELOG entry is added (if required)
• [x] FCM (final commit message) is posted
  • [x] and approved
• [x] Review is completed and changes are approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] Draft: prefix is removed
  • [x] All temporary labels are removed

[ilslv]

FCM

Fix input value coercion with defaults (#1080, #1073)