[release/3.1] Do not null ref or throw index out of range exception when deserializing to char.

src/System.Text.Json/src/System/Text/Json/Serialization/Converters/JsonValueConverterChar.cs

@@ -10,7 +10,12 @@ internal sealed class JsonConverterChar : JsonConverter<char>
     {
         public override char Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
-            return reader.GetString()[0];
+            string str = reader.GetString();
+            if (string.IsNullOrEmpty(str))
+            {
+                throw ThrowHelper.GetInvalidOperationException_ExpectedChar(reader.TokenType);
+            }
+            return str[0];
         }
 
         public override void Write(Utf8JsonWriter writer, char value, JsonSerializerOptions options)

src/System.Text.Json/src/System/Text/Json/ThrowHelper.cs

@@ -592,6 +592,11 @@ public static FormatException GetFormatException(DataType dateType)
             ex.Source = ExceptionSourceValueToRethrowAsJsonException;
             return ex;
         }
+
+        public static InvalidOperationException GetInvalidOperationException_ExpectedChar(JsonTokenType tokenType)
+        {
+            return GetInvalidOperationException("char", tokenType);
+        }
     }
 
     internal enum ExceptionResource

src/System.Text.Json/tests/Serialization/Null.ReadTests.cs

@@ -152,5 +152,16 @@ public static void NullAcceptsLeadingAndTrailingTrivia()
                 Assert.Null(obj);
             }
         }
+
+        [Fact]
+        public static void NullReadTestChar()
+        {
+            Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<char>("null"));
+            Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<char>("\"\""));
+            Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<char>(""));   // Empty JSON is invalid
+            Assert.Throws<JsonException>(() => JsonSerializer.Deserialize<char>("1234")); // Can't convert a JSON number to JSON string/char
+            Assert.Equal('a', JsonSerializer.Deserialize<char>("\"a\""));
+            Assert.Equal('Y', JsonSerializer.Deserialize<char>("\"\u0059\""));
+        }
     }
 }