Fix #19297 - fixing broken list after adding empty list (#19299)

* Update lists.nim

* Update tlists.nim

* removed check `if b.tail != nil`

The tail of the list being null it is still possible to retrieve its end by going through all nodes from the head. So checking for null from `b.tail` is unnecessary. However, setting `a.tail = b.tail` only if `a.head != nil`, so you don't break a good list with an already broken one.

lib/pure/collections/lists.nim

@@ -530,11 +530,12 @@ proc addMoved*[T](a, b: var SinglyLinkedList[T]) {.since: (1, 5, 1).} =
         ci
     assert s == [0, 1, 0, 1, 0, 1]
 
-  if a.tail != nil:
+  if b.head != nil:
-    a.tail.next = b.head
-  a.tail = b.tail
     if a.head == nil:
       a.head = b.head
+    else:
+      a.tail.next = b.head
+    a.tail = b.tail
   if a.addr != b.addr:
     b.head = nil
     b.tail = nil

tests/stdlib/tlists.nim

@@ -234,5 +234,17 @@ template main =
     doAssert l.remove(l.head) == true
     doAssert l.toSeq == []
   
+  block issue19297: # add (appends a shallow copy)
+    var a: SinglyLinkedList[int]
+    var b: SinglyLinkedList[int]
+
+    doAssert a.toSeq == @[]
+    a.add(1)
+    doAssert a.toSeq == @[1]
+    a.add(b)
+    doAssert a.toSeq == @[1]
+    a.add(2)
+    doAssert a.toSeq == @[1, 2]
+
 static: main()
 main()