Fix TestXmlReferenceResolver semantics for missing files

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/TestXmlReferenceResolver.cs

@@ -24,18 +24,28 @@ public override int GetHashCode()
             return RuntimeHelpers.GetHashCode(this);
         }
 
-        public override Stream? OpenRead(string resolvedPath)
+        public override Stream OpenRead(string resolvedPath)
         {
+            if (resolvedPath is null)
+            {
+                throw new ArgumentNullException(nameof(resolvedPath));
+            }
+
             if (!XmlReferences.TryGetValue(resolvedPath, out var content))
             {
-                return null;
+                throw new IOException($"Unable to read XML file: {resolvedPath}");
             }
 
             return new MemoryStream(Encoding.UTF8.GetBytes(content));
         }
 
-        public override string ResolveReference(string path, string baseFilePath)
+        public override string? ResolveReference(string path, string baseFilePath)
         {
+            if (!XmlReferences.ContainsKey(path))
+            {
+                return null;
+            }
+
             return path;
         }
     }