Fix NRE when printing directive without locations (#300)

Author: sungam3r

src/GraphQLParser.Tests/Visitors/SDLPrinterTests.cs

@@ -823,6 +823,46 @@ public void UTF8_MemoryStream_Runs_Synchronously()
         printer.PrintAsync(document, writer).IsCompletedSuccessfully.ShouldBeTrue();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void Printer_Should_Print_DirectiveDefinition_Without_Locations(bool eachDirectiveLocationOnNewLine)
+    {
+        var printer = new SDLPrinter(new SDLPrinterOptions
+        {
+            EachDirectiveLocationOnNewLine = eachDirectiveLocationOnNewLine,
+        });
+        var document = new GraphQLDocument
+        {
+            Definitions = new List<ASTNode>
+            {
+                new GraphQLDirectiveDefinition
+                {
+                    Name = new GraphQLName("null_locations"),
+                    Locations = new GraphQLDirectiveLocations() // Items is null
+                },
+                new GraphQLDirectiveDefinition
+                {
+                    Name = new GraphQLName("empty_locations"),
+                    Locations = new GraphQLDirectiveLocations() { Items = new List<DirectiveLocation>() } // Items is empty
+                },
+                new GraphQLScalarTypeDefinition
+                {
+                    Name = new GraphQLName("AAA")
+                }
+            }
+        };
+
+        var actual = printer.Print(document);
+        actual.ShouldBe("""
+            directive @null_locations on
+
+            directive @empty_locations on
+
+            scalar AAA
+            """);
+    }
+
     [Theory]
     [InlineData("query a { name }")]
     [InlineData("directive @skip(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT")]

src/GraphQLParser/Visitors/SDLPrinter.cs

@@ -372,24 +372,31 @@ protected override async ValueTask VisitDirectiveLocationsAsync(GraphQLDirective
         if (Options.EachDirectiveLocationOnNewLine)
         {
             await context.WriteAsync(" on").ConfigureAwait(false);
-            await context.WriteLineAsync().ConfigureAwait(false);
-            for (int i = 0; i < directiveLocations.Items.Count; ++i)
+            if (directiveLocations.Items?.Count > 0) // should always be true but may be negligently uninitialized
             {
-                await WriteIndentAsync(context).ConfigureAwait(false);
-                await context.WriteAsync("| ").ConfigureAwait(false);
-                await context.WriteAsync(GetDirectiveLocation(directiveLocations.Items[i])).ConfigureAwait(false);
-                if (i < directiveLocations.Items.Count - 1)
-                    await context.WriteLineAsync().ConfigureAwait(false);
+                await context.WriteLineAsync().ConfigureAwait(false);
+                for (int i = 0; i < directiveLocations.Items.Count; ++i)
+                {
+                    await WriteIndentAsync(context).ConfigureAwait(false);
+                    await context.WriteAsync("| ").ConfigureAwait(false);
+                    await context.WriteAsync(GetDirectiveLocation(directiveLocations.Items[i])).ConfigureAwait(false);
+                    if (i < directiveLocations.Items.Count - 1)
+                        await context.WriteLineAsync().ConfigureAwait(false);
+                }
             }
         }
         else
         {
-            await context.WriteAsync(" on ").ConfigureAwait(false);
-            for (int i = 0; i < directiveLocations.Items.Count; ++i)
+            await context.WriteAsync(" on").ConfigureAwait(false);
+            if (directiveLocations.Items?.Count > 0) // should always be true but may be negligently uninitialized
             {
-                await context.WriteAsync(GetDirectiveLocation(directiveLocations.Items[i])).ConfigureAwait(false);
-                if (i < directiveLocations.Items.Count - 1)
-                    await context.WriteAsync(" | ").ConfigureAwait(false);
+                await context.WriteAsync(" ").ConfigureAwait(false);
+                for (int i = 0; i < directiveLocations.Items.Count; ++i)
+                {
+                    await context.WriteAsync(GetDirectiveLocation(directiveLocations.Items[i])).ConfigureAwait(false);
+                    if (i < directiveLocations.Items.Count - 1)
+                        await context.WriteAsync(" | ").ConfigureAwait(false);
+                }
             }
         }
     }