code: func effectiveType(varType Type, varDef *ast.VariableDefinition) Type {
  	if varDef.DefaultValue == nil {
  		return varType
  	}
  	if _, ok := varType.(*NonNull); ok {
  		return varType
  	}
  	return NewNonNull(varType)
  }
docs: // If a variable definition has a default value, it's effectively non-null.