fix exception when a token is just a prefix

Author: jonsequitur

src/System.CommandLine.Tests/ParserTests.cs

@@ -148,6 +148,21 @@ public void When_no_options_are_specified_then_an_error_is_returned()
                   .Be("You must specify at least one option or command.");
         }
 
+        [Theory]
+        [InlineData("-")]
+        [InlineData("/")]
+        public void When_a_token_is_just_a_prefix_then_an_error_is_returned(string prefix)
+        {
+            var parser = new Parser(new RootCommand());
+
+            var result = parser.Parse(prefix);
+
+            result.Errors
+                  .Select(e => e.Message)
+                  .Should()
+                  .Contain(ValidationMessages.Instance.UnrecognizedCommandOrArgument(prefix));
+        }
+
         [Fact]
         public void Two_options_cannot_have_conflicting_aliases()
         {

src/System.CommandLine/Parsing/StringExtensions.cs

@@ -171,7 +171,7 @@ internal static TokenizeResult Tokenize(
 
             return new TokenizeResult(tokenList, errorList);
 
-            bool CanBeUnbundled(string arg, out IEnumerable<string> replacement)
+            bool CanBeUnbundled(string arg, out IReadOnlyList<string> replacement)
             {
                 replacement = null;
 
@@ -208,8 +208,14 @@ void AddRestValue(List<string> list, string rest)
                     }
                 }
 
-                bool TryUnbundle(string arg, out IEnumerable<string> replacement)
+                bool TryUnbundle(string arg, out IReadOnlyList<string> replacement)
                 {
+                    if (arg == string.Empty)
+                    {
+                        replacement = null;
+                        return false;
+                    }
+
                     var lastTokenHasArgument = false;
                     var builder = new List<string>();
                     for (var i = 0; i < arg.Length; i++)