Add isNil for Preserve

Not a ref type, but add `isNil` for checking if a Perserve is
zero-initialized.

src/preserves.nim

@@ -59,6 +59,10 @@ proc assertValid*(prs: Preserve) =
   else:
     discard
 
+proc isNil*(prs: Preserve): bool =
+  ## Check if ``prs`` is equivalent to the zero-initialized ``Preserve``.
+  prs.kind == pkBoolean and prs.bool == false
+
 proc `<`(x, y: string | seq[byte]): bool =
   for i in 0 .. min(x.high, y.high):
     if x[i] < y[i]: