Annotate TryLookupByType for nullability (#3719)

* Annotate TryLookupByType for nullability

* Address review comments

- Apply review comments from #3662.
- Tweak implementation.

* Fix formatting

Add missing space.

---------

Co-authored-by: Wajahat Ahmad <67877858+wajahati@users.noreply.github.com>

Author: martincostello

src/Swashbuckle.AspNetCore.SwaggerGen/SwaggerGenerator/SchemaRepository.cs

@@ -8,7 +8,7 @@ public class SchemaRepository(string documentName = null)
 
     public string DocumentName { get; } = documentName;
 
-    public Dictionary<string, IOpenApiSchema> Schemas { get; private set; } = [];
+    public Dictionary<string, IOpenApiSchema> Schemas { get; } = [];
 
     public void RegisterType(Type type, string schemaId)
     {
@@ -24,16 +24,17 @@ public void RegisterType(Type type, string schemaId)
         _reservedIds.Add(type, schemaId);
     }
 
-    public bool TryLookupByType(Type type, out OpenApiSchemaReference referenceSchema)
+    public bool TryLookupByType(Type type, [System.Diagnostics.CodeAnalysis.NotNullWhen(true)] out OpenApiSchemaReference referenceSchema)
     {
-        if (_reservedIds.TryGetValue(type, out string schemaId))
+        referenceSchema = null;
+        bool result = _reservedIds.TryGetValue(type, out string schemaId);
+
+        if (result)
         {
             referenceSchema = new OpenApiSchemaReference(schemaId);
-            return true;
         }
 
-        referenceSchema = null;
-        return false;
+        return result;
     }
 
     public OpenApiSchemaReference AddDefinition(string schemaId, OpenApiSchema schema)