Loss of Idempotence in JsonSerDe Round Trip for AggregateCall and RexNode

JsonSerDeTestUtil#testJsonRoundTrip only checks the equality between spec and deserialized object. Some corner cases are detected when serializing the deserialized object again.

static <T> T testJsonRoundTrip(SerdeContext serdeContext, T spec, Class<T> clazz)
            throws IOException {
        String actualJson = toJson(serdeContext, spec);
        T actual = toObject(serdeContext, actualJson, clazz);

        assertThat(actual).isEqualTo(spec);
        assertThat(actualJson).isEqualTo(toJson(serdeContext, actual)); // this will eval some corner cases
        return actual;
    }

The discovered corner cases are listed as follows.

1. SerDe for AggregateCall

When deserializing the aggregate call, we should check the JsonNodeType to avoid converting null to "null" string.
https://github.com/apache/flink/blob/f9b3e0b7bc0432001b4a197539a0712b16e0b33b/flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/plan/nodes/exec/serde/AggregateCallJsonDeserializer.java#L64

Suggested Fix

JsonNode nameNode = jsonNode.required(FIELD_NAME_NAME);
final String name = JsonNodeType.NULL ? null : nameNode.asText();

2. SerDe for RexNode

RexNodeJsonSerdeTest#testSystemFunction should create the temporary system function instead of the temporary catalog function.
https://github.com/apache/flink/blob/f9b3e0b7bc0432001b4a197539a0712b16e0b33b/flink-table/flink-table-planner/src/test/java/org/apache/flink/table/planner/plan/nodes/exec/serde/RexNodeJsonSerdeTest.java#L209

Suggested Fix

Use functionCatalog#registerTemporarySystemFunction to test.

3. About RexLiteral type

RexNodeJsonSerdeTest#testRexNodeSerde has a test spec as follows

//This will create the literal with DOUBLE as the literal type, and DECIMAL as the broad type of this literal. You can refer to Calcite for more details
rexBuilder.makeExactLiteral(BigDecimal.valueOf(Double.MAX_VALUE), FACTORY.createSqlType(SqlTypeName.DOUBLE))

The RexNodeJsonSerializer uses `typeName`(which is DECIMAL) as the literal's type, as a result, the rel data type is serialized as double, but the value is serialized as a string (in case lost the precision)
https://github.com/apache/flink/blob/f9b3e0b7bc0432001b4a197539a0712b16e0b33b/flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/plan/nodes/exec/serde/RexNodeJsonSerializer.java#L197

And then, during the deserialization, according to the JSON, the deserialized literal will assign DOUBLE as the literal type and the broad type of the literal.
This will cause the comparison failure

expected: {"kind": "LITERAL", "value": "1.7976931348623157E+308"}
actual: {"kind": "LITERAL", "value": 1.7976931348623157E+308}

Suggested Fix

SARG is a special case and can be coped first, and for the rest type, we can use literal.getType().getSqlTypeName() instead of literal.getTypeName().

// first cope with SARG type
if (literal.getTypeName() == SARG) {
    serializeSargValue(
        (Sarg<?>) value, literal.getType().getSqlTypeName(), gen, serializerProvider);
} else {
    serializeLiteralValue(
        value,
        literal.getType().getSqlTypeName(),
        gen,
        serializerProvider);
}