nil in string concats does not produce crashes anymore

tests/system/tnilconcats.nim

@@ -0,0 +1,25 @@
+discard """
+  output: '''@[nil, nil, nil, nil, nil, nil, nil, "meh"]'''
+  exitcode: "0"
+"""
+
+when true:
+  var ab: string
+  ab &= "more"
+
+  doAssert ab == "more"
+
+  var x: seq[string]
+
+  setLen(x, 7)
+
+  x.add "meh"
+
+  var s: string
+  var z = "abc"
+  var zz: string
+  s &= "foo" & z & zz
+
+  doAssert s == "fooabc"
+
+  echo x