Nim string "" is not converted automatically to nil C string

[Stefan_Salewski]

I heard about that Nim strings will become value types.

With latest Nim devel even nimble breaks, so it was clear that current gintro 
would not survive :-)
    
    
    nimblepkg/packageinfo.nim(110, 25) Error: type mismatch: got <JsonNode, 
string, nil>
    but expected one of:
    proc optionalField(obj: JsonNode; name: string; default = ""): string
      first type mismatch at position: 3
      required type: string
      but expression 'nil' is of type: nil
    
    
    Run

Indeed I had expected that there is no automatic conversion from empty Nim 
strings to NULL passed to C libs:
    
    
    proc clib(s: cstring) =
      echo s.isNil
      echo s.len
      echo cast[int8](s[0])
      echo cast[int](s)
    
    proc main =
      clib("")
    
    main()
    
    
    Run

At least GTK generally likes to get NULL pointers, and not cstrings with first 
char being value 0x0.

So I think I may apply a proc converting Nim "" to nil when C lib is called. 
For the rare case when user really wants to pass an empty cstring to C lib he 
may pass "x0" Nim string, maybe after calling setLen(1).