create new deferred grouped field set records lazily

[yaacovCR]

Before #3984, we could not do this because of problems with empty defers. Now, it's a safe optimization.

Consider:

      query {
        alwaysThrows
        ... @defer(name: "WillBeEmpty") {
          someField
        }
      }

Before the current change, when alwaysThrows throws, the deferred grouped field set for someField would be found to be in the "blast radius", and deferred fragment WillBeEmpty would be filtered. This would yield:

    {
      "data": {
        "alwaysThrows": null,
      },
      "errors": [ ... ]
    }

Prior to #3984, if we had made the current change and created the deferred grouped field set for someField lazily, someField would not be found to be in the "blast radius" -- because it would not have been created yet -- and WillBeEmpty would not have been filtered.

However, with #3984, WillBeEmpty is not sent because all empty deferred fragments are never sent and do not require filtering. This is now a safe change.

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[yaacovCR]

Note that there is a tweak/fix for #3984 in #3993 for subsequent results.

[yaacovCR]

Actually, wait => this is still not safe!

What if a deferred fragment that should be filtered is never filtered, but it wasn't otherwise empty?

Consider:

      query {
        someRootField {
          alwaysThrows
        }
        ... @defer(name: "WillNotBeEmpty") {
          someRootField {
            someNestedField
          }
          someOtherRootField
        }
      }

By the time alwaysThrows throws, a deferred grouped field set could be created for someOtherRootField (if someRootField itself is async, for example) and so deferred fragment WillNotBeEmpty will not be empty. But it cannot be delivered, because it should be found to be in the blast radius if alwaysThrows by someRootField.

[yaacovCR]

TODO: add test for above