Fixes ropes regressions due to the not-nil strings (#8687)

tests/stdlib/tropes.nim

@@ -0,0 +1,36 @@
+discard """
+  file: "tropes.nim"
+  output: '''0
+3
+
+123
+3
+6
+123
+123456
+2
+3'''
+"""
+import ropes
+
+var
+  r1 = rope("")
+  r2 = rope("123")
+
+echo r1.len
+echo r2.len
+
+echo r1
+echo r2
+
+r1.add("123")
+r2.add("456")
+
+echo r1.len
+echo r2.len
+
+echo r1
+echo r2
+
+echo r1[1]
+echo r2[2]