Messages: Produce: "Keep contents" prevents new dummy data from being generated

[Haarolean]

Since #2672
Setup:

1. Topic "test"
2. Two JSON schemas, "test-key", "test-value":

{
	"schema": "http://json-schema.org/draft-07/schema#",
	"id": "http://example.com/myURI.schema.json",
	"title": "TestRecord",
	"type": "object",
	"properties": {
		"f1": {
			"type": "integer"
		}
	}
}

Steps to reproduce:

1. Click "produce message". Observe that dummy data is generated for value
2. Produce a message
3. Open produce message again. No new data has been generated

#3901