allow using merge directive without keyArg argument if resolver takes 0 arguments

[dimatillck]

Description

Allows using @merge directive without keyArg argument if resolver takes 0 arguments
Related #4336

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[changeset-bot]

🦋 Changeset detected

Latest commit: 6b91b09

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@graphql-tools/stitching-directives	Patch
federation-benchmark	Patch

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[dimatillck]

The failed test is unrelated, please rerun it

[yaacovCR]

But does merging work? Do we have a test elsewhere? I think it does and we do, but offhand I am not sure where.

[dimatillck]

Hi, @yaacovCR , I did a manual test and it works. I didn't find the test so I added it, please check it

[dimatillck]

@yaacovCR well, now I can confirm that merging doesn't work well if querying not all the fields from the type. I've updated the test case to fail.