Guard against calling split with an empty string as a separator. Fixes #5119

tests/stdlib/tsplit2.nim

@@ -0,0 +1,19 @@
+discard """
+  file: "tsplit2.nim"
+  output: "true"
+"""
+import strutils
+
+var s = ""
+for w in split("|abc|xy|z", {'|'}):
+  s.add("#")
+  s.add(w)
+
+try:
+  discard "hello".split("")
+  echo "false"
+except ValueError:
+  echo "true"
+
+#OUT true
+