Fix error when printing a FieldDefinition AST node without arguments field

[sullvn]

I encountered a runtime type error while printing some hand-constructed FieldDefinition AST nodes which conform to the type spec.

Problem

Type spec - declaring it's optional:

graphql-js/src/language/ast.js

Line 444 in 26c9874

+arguments?: $ReadOnlyArray<InputValueDefinitionNode>,

Printing implementation - requiring it:

graphql-js/src/language/printer.js

Line 139 in 7874229

(args.every(arg => arg.indexOf('\n') === -1)

Solution

This PR fixes this discrepancy in the implementation with a default value.

https://github.com/awfulaxolotl/graphql-js/blob/df56ba3eadbfbc282ae6262927dfaeb162662158/src/language/printer.js#L139

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[IvanGoncharov]

@awfulaxolotl Sorry for the delay.
I reviewed our printer implementation and found another place with an exact same problem.
So I extracted code in question into hasMultilineItem function and reuse it in both places.
I pushed my changes on top of your PR and it would be great if you have time to review it.

@mjmahone Can you please review?

[IvanGoncharov]

@mjmahone General issue here is that printer.js is not covered by Flow but I will try to address this in the separate PR.

[sullvn]

@IvanGoncharov Nice catch. Your extended fix looks good to me! Looking forward to printer.js being covered with Flow.

[IvanGoncharov]

@mjmahone Can you please review?

[mjmahone]

Thanks you for finding and fixing this bug! Sorry for my review delay, but @IvanGoncharov seems to have helped move this fix along well. Thank you both!