Scripting: redis.NIL to return nil bulk replies.

Lua arrays can't contain nil elements (see
http://www.lua.org/pil/19.1.html for more information), so Lua scripts
were not able to return a multi-bulk reply containing nil bulk
elements inside.

This commit introduces a special conversion: a table with just
a "nilbulk" field set to a boolean value is converted by Redis as a nil
bulk reply, but at the same time for Lua this type is not a "nil" so can
be used inside Lua arrays.

This type is also assigned to redis.NIL, so the following two forms
are equivalent and will be able to return a nil bulk reply as second
element of a three elements array:

    EVAL "return {1,redis.NIL,3}" 0
    EVAL "return {1,{nilbulk=true},3}" 0

The result in redis-cli will be:

    1) (integer) 1
    2) (nil)
    3) (integer) 3

tests/unit/scripting.tcl

@@ -30,6 +30,10 @@ start_server {tags {"scripting"}} {
         set _ $e
     } {this is an error}
 
+    test {EVAL - Lua nil reply -> Redis protocol type conversion} {
+        r eval {return {1,redis.NIL,{nilbulk=true},4}} 0
+    } {1 {} {} 4}
+
     test {EVAL - Lua table -> Redis protocol type conversion} {
         r eval {return {1,2,3,'ciao',{1,2}}} 0
     } {1 2 3 ciao {1 2}}