Ensure `FileDescriptor::edition_` is correctly initialized when built from `FileDescriptorProto`

When `DescriptorBuilder::BuildFileImpl` is called, `alloc.AllocateArray` appears to be allocating an uninitialized object. This means every field inside FileDescriptor should be initialized, and edition_ may be ignored if not present. This can then lead to a SEGFAULT later in `FileDescriptor::edition()`, with a check of `edition_ == nullptr ? anti_hyrum_string : *edition_;` potentially dereferencing `edition_` when it's uninitialized.

PiperOrigin-RevId: 549712519

src/google/protobuf/descriptor.cc

@@ -5748,6 +5748,8 @@ FileDescriptor* DescriptorBuilder::BuildFileImpl(
   }
   if (proto.has_edition()) {
     file_->edition_ = alloc.AllocateStrings(proto.edition());
+  } else {
+    file_->edition_ = nullptr;
   }
 
   result->name_ = alloc.AllocateStrings(proto.name());