Fix an empty JSON map append using _dummy subcolumn

[jkaflik]

if map is empty, we need to create an empty Tuple to make sure subcolumns protocol is happy
_dummy is a ClickHouse internal name for empty Tuple subcolumn
it has the same effect as INSERT INTO single_json_type_table VALUES ('{}');

resolves #1113

[leodido]

I've also implemented a fix @jkaflik.

Well, I've implemented 2. I'm gonna write them here in case you find them useful.

The first fix is to replicate the behavior for the string {} in the map case (so in the appendStructOrMap function).

// Handle empty maps
if jCol.columns == nil && vData.Len() == 0 {
	jCol.encoding = 1
	jCol.columns = append(jCol.columns, &JSONValue{Interface: &String{}})
	return jCol.columns[0].AppendRow(nil) // `{}` input also works
}

The alternative fix that I've developed is (always in the same spot in the appendStructOrMap function).

if jCol.columns == nil && vData.Len() == 0 {
	jCol.upsertValue("_dummy", "Int8")
	jCol.insertEmptyColumn("_dummy")
	return nil
}

In terms of visual results they're totally equivalent to the one you've implemented. Ie, col4=map[_dummy:0].

In case you prefer any of my 2 solutions you just tell me and I'll send it with a PR. Or feel free to pick any of them (co-authorship would be appreciated :)).

[jkaflik]

@leodido

Thanks. The second one is cleaner but has a slight overhead on type resolve. In that case, I would not bother about it since the entire JSON implementation is costly anyway.

Feel free to submit your second proposal, but please add a few lines of comments so anyone who reviews it later is not frustrated around this hack. :)
Also, please copy and paste the test case.
If you submit it today, I am ok to merge yours 🚀

[leodido]

On it!

[leodido]

Here it is: #1116

[jkaflik]

Closed in favour of #1116