fix MemoryPackReader.PeekIsNull

Author: neuecc

src/MemoryPack.Core/MemoryPackReader.cs

@@ -297,7 +297,8 @@ public bool TryReadCollectionHeader(out int length)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public bool PeekIsNull()
     {
-        return TryPeekObjectHeader(out _);
+        var code = GetSpanReference(1);
+        return code == MemoryPackCode.NullObject;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

src/MemoryPack.Unity/Assets/Plugins/MemoryPack/Runtime/MemoryPack.Core/MemoryPackReader.cs

@@ -306,7 +306,8 @@ public bool TryReadCollectionHeader(out int length)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public bool PeekIsNull()
     {
-        return TryPeekObjectHeader(out _);
+        var code = GetSpanReference(1);
+        return code == MemoryPackCode.NullObject;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

tests/MemoryPack.Tests/ReaderTest.cs

@@ -167,4 +167,15 @@ public void ValidateInvalidLengthTest()
         catch (MemoryPackSerializationException) { }
     }
 
+    [Fact]
+    public void PeekIsNull()
+    {
+        var bin = MemoryPackSerializer.Serialize<string>(null);
+
+        using var state = MemoryPackReaderOptionalStatePool.Rent(null);
+        var reader = new MemoryPackReader(bin, state);
+
+        var isNull = reader.PeekIsNull();
+        isNull.Should().BeTrue();
+    }
 }