fix(compiler): InputValueDefinition::is_required() returns false if there is a default value

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Now argument.is_required() == true only if the type is non-null and there is no default value, meaning a value must be provided when it's used. This is how the spec describes required arguments.

This seems more useful as the previous implementation can already be achieved by checking argument.ty.is_non_null().

It's kind of a fix, kind of just a change, kind of breaking...

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I didn't find any!