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

**Is your feature request related to a problem? Please describe.**

Currently `stitchingDirectivesValidator` doesn't allow to use `@merge` directive without `keyArg` argument if resolver takes 0 arguments.

**Describe the solution you'd like**

Allow using `@merge` directive without `keyArg` argument if resolver takes 0 arguments

**Additional context**

Allow such use-case will be helpful for situations when there is a global object with fields resolved by different subschemas. An example of such use-case could be some global Stats Object which has different stat fields that are resolved by different subschemas, so there is nothing to pass to the merge resolver

Locate `stitchingDirectivesValidator` and trace how it validates `@merge` directives without `keyArg`. Check the zero-argument resolver case described in the issue, then verify that a merge directive without `keyArg` is accepted only when the resolver takes no arguments and that existing validation behavior remains unchanged.

Written by the indexing model from the issue text.