Fix parsing of certain LGB files

These have unknown instance objects, and were tripping up our parsing. I
don't know what these are yet, but I added them anyway.

I also made sure that the parser doesn't return None for technically
valid but empty LGB files.

src/layer/exit_range.rs

@@ -10,6 +10,7 @@ use super::TriggerBoxInstanceObject;
 #[derive(Debug, PartialEq)]
 pub enum ExitType {
     ZoneLine = 0x1,
+    Unk = 0x2, // seen in bg/ex5/02_ykt_y6/fld/y6f1/level/planmap.lgb
 }
 
 #[binread]

src/layer/mod.rs

@@ -298,6 +298,8 @@ pub enum LayerEntryType {
     StableChocobo = 0x4F,
     MaxAssetType = 0x50,
     Unk1 = 90,
+    Unk2 = 86, // seen in bg/ex5/02_ykt_y6/fld/y6f1/level/bg.lgb
+    Unk3 = 89, // seen in bg/ffxiv/sea_s1/fld/s1f3/level/planevent.lgb
 }
 
 #[binread]
@@ -362,6 +364,10 @@ pub enum LayerEntryData {
     FateRange(FateRangeInstanceObject),
     #[br(pre_assert(*magic == LayerEntryType::Unk1))]
     Unk1(),
+    #[br(pre_assert(*magic == LayerEntryType::Unk2))]
+    Unk2(),
+    #[br(pre_assert(*magic == LayerEntryType::Unk3))]
+    Unk3(),
 }
 
 #[binread]
@@ -659,9 +665,11 @@ impl LayerGroup {
 
         let chunk_header =
             LayerChunkHeader::read_le_args(&mut cursor, (&chunk_string_heap,)).unwrap();
-
         if chunk_header.chunk_size <= 0 {
-            return None;
+            return Some(LayerGroup {
+                file_id: file_header.file_id,
+                chunks: Vec::new(),
+            });
         }
 
         let old_pos = cursor.position();
@@ -706,8 +714,7 @@ impl LayerGroup {
                     let start = cursor.stream_position().unwrap();
                     let string_heap = StringHeap::from(start);
 
-                    objects
+                    objects.push(InstanceObject::read_le_args(&mut cursor, (&string_heap,)).ok()?);
-                        .push(InstanceObject::read_le_args(&mut cursor, (&string_heap,)).ok()?);
                 }
             }