--nilseqs is now a deprecated noop (#17211)

* --nilseqs is now a deprecated noop

* fix tests; fix: future => sugar

tests/niminaction/Chapter1/various1.nim

@@ -32,7 +32,7 @@ block: # Block added due to clash.
   let dog = Dog()
   dog.bark() #<2>
 
-import sequtils, future, strutils
+import sequtils, sugar, strutils
 let list = @["Dominik Picheta", "Andreas Rumpf", "Desmond Hume"]
 list.map(
   (x: string) -> (string, string) => (x.split[0], x.split[1])

tests/niminaction/Chapter2/resultaccept.nim

@@ -22,7 +22,7 @@ proc resultVar2: string =
   result.add("returned")
 
 doAssert implicit() == "I will be returned"
-doAssert discarded() == nil
+doAssert discarded().len == 0
 doAssert explicit() == "I will be returned"
 doAssert resultVar() == "I will be returned"
 doAssert resultVar2() == "I will be returned"

tests/niminaction/Chapter2/various2.nim

@@ -140,7 +140,7 @@ let numbers = @[1, 2, 3, 4, 5, 6]
 let odd = filter(numbers, proc (x: int): bool = x mod 2 != 0)
 doAssert odd == @[1, 3, 5]
 
-import sequtils, future
+import sequtils, sugar
 let numbers1 = @[1, 2, 3, 4, 5, 6]
 let odd1 = filter(numbers1, (x: int) -> bool => x mod 2 != 0)
 assert odd1 == @[1, 3, 5]
@@ -149,7 +149,7 @@ proc isValid(x: int, validator: proc (x: int): bool) =
   if validator(x): echo(x, " is valid")
   else: echo(x, " is NOT valid")
 
-import future
+import sugar
 proc isValid2(x: int, validator: (x: int) -> bool) =
   if validator(x): echo(x, " is valid")
   else: echo(x, " is NOT valid")
@@ -193,7 +193,7 @@ doAssertRaises(IndexDefect):
 
 block:
   var list = newSeq[string](3)
-  assert list[0] == nil
+  assert list[0].len == 0
   list[0] = "Foo"
   list[1] = "Bar"
   list[2] = "Baz"