make more tests green; system.repr does not produce 'nil' for strings and seqs anymore

tests/async/tfuturevar.nim

@@ -35,7 +35,7 @@ proc main() {.async.} =
   fut = newFutureVar[string]()
   let retFut = failureTest(fut, true)
   yield retFut
-  doAssert(fut.read().isNil)
+  doAssert(fut.read().len == 0)
   doAssert(fut.finished)
 
   fut = newFutureVar[string]()

tests/ccgbugs/tmissinginit.nim

@@ -3,8 +3,8 @@ discard """
 0
 0
 0
-[[a = nil,
-b = nil]]'''
+[[a = "",
+b = []]]'''
 """
 
 # bug #1475

tests/gc/thavlak.nim

@@ -245,7 +245,7 @@ proc findLoops(self: var HavlakLoopFinder): int =
   #     - the list of backedges (backPreds) or
   #     - the list of non-backedges (nonBackPreds)
   #
-  for w in 0 .. <size:
+  for w in 0 ..< size:
     header[w] = 0
     types[w]  = BB_NONHEADER