Fix #137: empty anonymous records

Author: Tarmil

src/FSharp.SystemTextJson/Record.fs

@@ -33,7 +33,7 @@ type JsonRecordConverter<'T>(options: JsonSerializerOptions, fsOptions: JsonFSha
         let allPublic =
             recordType.GetProperties(BindingFlags.Instance ||| BindingFlags.Public)
         let all =
-            if fields[0].GetGetMethod(true).IsPublic then
+            if fields.Length = 0 || fields[0].GetGetMethod(true).IsPublic then
                 allPublic
             else
                 Array.append fields allPublic

tests/FSharp.SystemTextJson.Tests/Test.Record.fs

@@ -162,6 +162,16 @@ module NonStruct =
         let actual = JsonSerializer.Serialize({| x = 1; y = "b" |}, options)
         Assert.Equal("""{"x":1,"y":"b"}""", actual)
 
+    [<Fact>]
+    let ``deserialize empty anonymous`` () =
+        let actual = JsonSerializer.Deserialize("{}", options)
+        Assert.Equal({|  |}, actual)
+
+    [<Fact>]
+    let ``serialize empty anonymous`` () =
+        let actual = JsonSerializer.Serialize({|  |}, options)
+        Assert.Equal("{}", actual)
+
     type PropName =
         { unnamedX: int
           [<JsonPropertyName "namedY">]