fix(reader): return diagnostic for empty streams

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>

Author: Copilot

src/Microsoft.OpenApi/Reader/OpenApiModelFactory.cs

@@ -212,9 +212,9 @@ public static ReadResult Parse(string input,
                                        OpenApiReaderSettings? settings = null)
         {
 #if NET6_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(input);
+            ArgumentException.ThrowIfNullOrEmpty(input);
 #else
-            if (input is null) throw new ArgumentNullException(nameof(input));
+            if (string.IsNullOrEmpty(input)) throw new ArgumentNullException(nameof(input));
 #endif
             format ??= InspectInputFormat(input);
             settings ??= new OpenApiReaderSettings();
@@ -246,9 +246,9 @@ public static ReadResult Parse(string input,
                                  OpenApiReaderSettings? settings = null) where T : IOpenApiElement
         {
 #if NET6_0_OR_GREATER
-            ArgumentNullException.ThrowIfNull(input);
+            ArgumentException.ThrowIfNullOrEmpty(input);
 #else
-            if (input is null) throw new ArgumentNullException(nameof(input));
+            if (string.IsNullOrEmpty(input)) throw new ArgumentNullException(nameof(input));
 #endif
             format ??= InspectInputFormat(input);
             settings ??= new OpenApiReaderSettings();
@@ -299,6 +299,20 @@ private static ReadResult InternalLoad(MemoryStream input, string format, OpenAp
             {
                 throw new InvalidOperationException("Loading external references are not supported when using synchronous methods.");
             }
+            if (input.Length == 0 || input.Position == input.Length)
+            {
+                var diagnostic = new OpenApiDiagnostic
+                {
+                    Format = format,
+                };
+                diagnostic.Errors.Add(new OpenApiError(null, $"Cannot parse the stream: {nameof(input)} is empty or contains no elements."));
+
+                return new()
+                {
+                    Document = null,
+                    Diagnostic = diagnostic,
+                };
+            }
             var location = new Uri(OpenApiConstants.BaseRegistryUri);
             var reader = settings.GetReader(format);
             var readResult = reader.Read(input, location, settings);