text: Title: [FEATURE] graphql not required handling
  Body: Currently when fields in graphql schemas are not required, schemathesis can send `null` to them.
  According to the graphql specs this is valid and it is useful to find bugs.
  
  But sometimes it would be easier to not send null values
  
  Is there a way to turn the null value sending behavior off? It would be nice to have a simple switch for it
  