Incorrect handling of empty optional UUID array header

[mxk]

What version of ogen are you using?

github.com/ogen-go/ogen v1.0.1-0.20240415140940-8c1c10db7ff2

Can this issue be reproduced with the latest version?

Yes

What did you do?

I generated client and server code from the following UUID header parameter:

"parameters": [
	{
		"name": "My-Array",
		"in": "header",
		"required": false,
		"schema": {
			"type": "array",
			"items": {
				"type": "string",
				"format": "uuid"
			}
		}
	}
]

What did you expect to see?

Invoking the API from the client using httptest server should work when this parameter is not specified in the request.

What did you see instead?

First, the client sends an empty string header for this parameter when it would be more efficient to omit the header since it's optional.

Second, the DecodeArray function on the server splits the string on commas, and then tries to decode the UUID using conv.ToUUID, which fails with invalid UUID length: 0 error.

There needs to be an explicit check on the server for an empty array, but the client should also avoid sending this header when it doesn't have a value.