feat(sterni/nix/utf8): allow decoding the empty string

Change-Id: I8de9cd28c822ac5befbcd16e118440cd13cd86e9

users/sterni/nix/utf8/default.nix

@@ -201,10 +201,10 @@ let
 
         # filter out all iteration steps without a codepoint value
         codepoint != null
-          # if we are at the iteration step of the input string, throw
+          # if we are at the iteration step of a non-empty input string, throw
           # an error if no codepoint was returned, as it indicates an incomplete
           # UTF-8 sequence.
-          || (stringIndex == stringLength - 1 && throw earlyEndMsg)
+          || (stringLength > 0 && stringIndex == stringLength - 1 && throw earlyEndMsg)
 
       ) iterResult
     );

users/sterni/nix/utf8/tests/default.nix

@@ -56,7 +56,6 @@ let
     utf8.decode (string.fromBytes (builtins.map int.fromHex l));
 
   testFailures = it "checks UTF-8 decoding failures" [
-    (assertThrows "emtpy bytestring throws" (utf8.decode ""))
     (assertThrows "truncated UTF-8 string throws" (hexDecode [ "F0" "9F" ]))
     # examples from The Unicode Standard
     (assertThrows "ill-formed: C0 AF" (hexDecode [ "C0" "AF" ]))
@@ -75,6 +74,7 @@ let
       ]);
 
   randomUnicode = [
+    "" # empty string should yield empty list
     "🥰👨‍👨‍👧‍👦🐈‍⬛👩🏽‍🦰"
     # https://kermitproject.org/utf8.html
     "ᚠᛇᚻ᛫ᛒᛦᚦ᛫ᚠᚱᚩᚠᚢᚱ᛫ᚠᛁᚱᚪ᛫ᚷᛖᚻᚹᛦᛚᚳᚢᛗ"