Empty check in shallow [backport] (#9676)

tests/seq/tseq.nim

@@ -170,6 +170,30 @@ block tshallowseq:
   xxx()
 
 
+block tshallowemptyseq:
+  proc test() =
+    var nilSeq: seq[int] = @[]
+    var emptySeq: seq[int] = newSeq[int]()
+    block:
+      var t = @[1,2,3]
+      shallow(nilSeq)
+      t = nilSeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallow(emptySeq)
+      t = emptySeq
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, nilSeq)
+      doAssert t == @[]
+    block:
+      var t = @[1,2,3]
+      shallowCopy(t, emptySeq)
+      doAssert t == @[]
+  test()
+
 
 import strutils
 block ttoseq: