Empty check in shallow [backport] (#9676)

lib/system.nim

@@ -4017,6 +4017,7 @@ proc shallow*[T](s: var seq[T]) {.noSideEffect, inline.} =
   ## marks a sequence `s` as `shallow`:idx:. Subsequent assignments will not
   ## perform deep copies of `s`. This is only useful for optimization
   ## purposes.
+  if s.len == 0: return
   when not defined(JS) and not defined(nimscript):
     var s = cast[PGenericSeq](s)
     s.reserved = s.reserved or seqShallowFlag