Handle empty index array in recover_fragments/7

When a stream is created but no segment has been flushed yet, the index
file exists on disk but contains no records after the header. The call
to `rabbitmq_stream_s3_array:at/3` on an empty binary crashes with
`badarg`.

Add a guard clause matching `<<>>` that returns a default empty
fragment, consistent with the `[]` branch in `init_manifest/2`.

Author: lukebakken

src/rabbitmq_stream_s3_log_manifest.erl

@@ -226,6 +226,18 @@ recover_fragments(IdxFile, Acc) ->
         IdxArray
     ).
 
+recover_fragments(
+    _Threshold0,
+    _SegmentFile,
+    SegmentOffset,
+    SeqNo0,
+    _NumChunks0,
+    Fragments0,
+    <<>>
+) ->
+    %% The index file exists but contains no records yet (e.g. the stream was
+    %% created but no segment has been flushed). Treat as an empty fragment.
+    {#fragment{segment_offset = SegmentOffset, seq_no = SeqNo0}, Fragments0};
 recover_fragments(
     Threshold0,
     SegmentFile,