Field argument change to/from `default_value = nil` not detected

[tgwizard]

I think this is partly because of

graphql-schema_comparator/lib/graphql/schema_comparator/diff/argument.rb

Lines 20 to 22 in 5053197

	if old_arg.default_value != new_arg.default_value
	changes << Changes::FieldArgumentDefaultChanged.new(type, field, old_arg, new_arg)
	end

I believe default_value is nil both for the case when it's not set and when it's set to nil, so we might need to add || old_arg.default_value? != new_arg.default_value?.

Then there's also

graphql-schema_comparator/lib/graphql/schema_comparator/changes.rb

Lines 453 to 460 in 5053197

	def message
	if old_argument.default_value.nil? || old_argument.default_value == :__no_default__
	"Default value `#{new_argument.default_value}` was added to argument `#{new_argument.graphql_name}` on field `#{type.graphql_definition}.#{field.graphql_name}`"
	else
	"Default value for argument `#{new_argument.graphql_name}` on field `#{type.graphql_definition}.#{field.name}` changed"\
	" from `#{old_argument.default_value}` to `#{new_argument.default_value}`"
	end
	end

Which renders poorly when default_value = nil (since nil becomes the empty string).

(Apologies for only submitting a bug report and not test cases or a PR fixing this. I had some weird issues getting the tests to pass deterministically locally...)