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

[dimatillck]

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