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.

tests/stdlib/tlists.nim

@@ -233,6 +233,18 @@ template main =
     doAssert l.toSeq == [1]
     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()