Clarify that the input to proto2::DynamicMessageFactory::GetPrototype should be non-null

PiperOrigin-RevId: 631938646

src/google/protobuf/dynamic_message.cc

@@ -620,6 +620,7 @@ DynamicMessageFactory::~DynamicMessageFactory() {
 }
 
 const Message* DynamicMessageFactory::GetPrototype(const Descriptor* type) {
+  ABSL_CHECK(type != nullptr);
   absl::MutexLock lock(&prototypes_mutex_);
   return GetPrototypeNoLock(type);
 }

src/google/protobuf/dynamic_message.h

@@ -108,8 +108,8 @@ class PROTOBUF_EXPORT DynamicMessageFactory : public MessageFactory {
   // prototype, so these must be destroyed before the DynamicMessageFactory
   // is destroyed.
   //
-  // The given descriptor must outlive the returned message, and hence must
+  // The given descriptor must be non-null and outlive the returned message, and
-  // outlive the DynamicMessageFactory.
+  // hence must outlive the DynamicMessageFactory.
   //
   // The method is thread-safe.
   const Message* GetPrototype(const Descriptor* type) override;