fix #2754 - Missing required IEnumerable argument causes runtime error

src/System.CommandLine.Tests/ParsingValidationTests.cs

@@ -214,6 +214,38 @@ public void When_a_required_argument_is_not_supplied_then_an_error_is_returned()
                   .Contain(e => e.Message == "Required argument missing for option: '-x'.");
         }
 
+        [Fact]
+        public void When_an_IEnumerable_argument_has_zero_minimum_arity_and_no_tokens_GetRequiredValue_returns_empty()
+        {
+            var argument = new Argument<IEnumerable<string>>("items");
+            var command = new RootCommand { argument };
+
+            var result = command.Parse("");
+
+            result.Errors.Should().BeEmpty();
+            result.GetRequiredValue(argument).Should().BeEmpty();
+        }
+
+        [Fact]
+        public void When_an_IEnumerable_argument_has_OneOrMore_arity_and_no_tokens_then_an_error_is_returned()
+        {
+            var argument = new Argument<IEnumerable<string>>("items")
+            {
+                Arity = ArgumentArity.OneOrMore
+            };
+            var command = new RootCommand { argument };
+
+            var result = command.Parse("");
+
+            result.Errors
+                  .Should()
+                  .ContainSingle()
+                  .Which
+                  .Message
+                  .Should()
+                  .Contain("items");
+        }
+
         [Fact]
         public void When_a_required_option_is_not_supplied_then_an_error_is_returned()
         {

src/System.CommandLine/Parsing/SymbolResult.cs

@@ -130,7 +130,7 @@ public T GetRequiredValue<T>(Argument<T> argument)
             => GetResult(argument) switch
             {
                 ArgumentResult argumentResult => argumentResult.GetValueOrDefault<T>(),
-                null => throw new InvalidOperationException($"{argument.Name} is required but was not provided."),
+                null => Argument<T>.CreateDefaultValue() ?? throw new InvalidOperationException($"{argument.Name} is required but was not provided."),
             };
 
         /// <inheritdoc cref="ParseResult.GetRequiredValue{T}(Option{T})"/>