Properly handle null values on arguments

[Maximilien-R]

Passing null values to arguments (or variables) aren't handled properly.

• Tartiflette version: 0.8.6
• Python version: 3.7.1
• Executed in docker: No
• Is a regression from a previous versions? No

SDL example:

type Query {
  hello(name: String = "Unknown"): String
  bye(name: String! = "Unknown"): String
}

Python:

@Resolver("Query.hello")
async def resolve_query_hello(parent, args, ctx, info):
    return args.get("name")


@Resolver("Query.bye")
async def resolve_query_bye(parent, args, ctx, info):
    return args.get("name")

Query:

query {
  hello(name: null)
} == {
  "data": {
    "hello": "Unknown"
  }
} != {
  "data": {
    "hello": null
  }
}