fixes #22153; UB calling allocCStringArray([""]) with --mm:refc (#24529)

fixes #22153

It's a problem for refc because you cannot index a nil string: i.e.
`[""]` is `{((NimStringDesc*) NIM_NIL)}` which cannot be indexed

(cherry picked from commit 9bb7e53e7f)

tests/objects/t20972.nim

@@ -13,3 +13,11 @@ var info = ForkedEpochInfo(kind: true)
 doAssert info.kind
 info.kind = false
 doAssert not info.kind
+
+block: # bug #22153
+  discard allocCStringArray([""])
+  discard allocCStringArray(["1234"])
+
+  var s = "1245"
+  s.add "1"
+  discard allocCStringArray([s])