Leaflet upload not mandatory when creating product data

[steinso2]

Subtask for Enhancement: #181

Currently when creating a product, its mandatory to upload a leaflet or smpc. Otherwise the product can't be saved.
With the (data integration) product interface, this will be split into two tasks:

1. The product is created with the interface (json)
2. the leaflet is uploaded in a second step manually.

Therefore, a product has to be saved without leaflet or smpc uploaded data. Otherwise the data integration won't work.