indexer/proto: correctly visit oneof (#6161)

See
https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md#to-iterate-over-all-oneofs
for more info.

This helps silencing some error within simple proto3 files

```
syntax = "proto3";

package moo;

message Bar {
  ...
  map<int32, int32> foo = 12;
}
```

which would yield
```
file_descriptor_walker.cc:262] Unexpected location vector [] while walking moo.proto
```

before this change.

Co-authored-by: Varun Mahajan <varun.mah@gmail.com>