Test and if necessary repair issues inserting `NULL` to get a default value when a column has an explicit `DEFAULT`

[theory]

From the ClickHouse Docs:

The expression expr is optional. If it is omitted, the column type must be specified explicitly and the default value will be 0 for numeric columns, '' (the empty string) for string columns, [] (the empty array) for array columns, 1970-01-01 for date columns, or NULL for nullable columns.

Currently the binary driver refuses to insert a NULL into a non-Nullable column, but the http driver allows it. Add tests with explicit DEFAULT values on the ClickHouse tables and ensure that the http driver does the right thing when inserting a NULL (with and without a corresponding constraint on the foreign Postgres table).

Determine whether the binary driver behavior should be the same, and if so remove this block and fix issues accordingly:

pg_clickhouse/src/binary.cpp

Lines 429 to 443 in a9f4fc0

	/*
	* Prevent insertion if the column isn't Nullable. This differs from the
	* http engine, which isn't aware of Nullable but just sends values off,
	* in which case ClickHouse inserts default values (e.g., 0 for numbers,
	* "" for strings, etc.).
	*
	* XXX Do we want to consider rejecting NULL inserts when the Postgres
	* `NOT NULL` constraint exits? And when it doesn't exist and the column
	* is not nullable, do we want to send the default value instead? Dropping
	* this block should do that.
	*/
	if (isnull && !nullable)
	throw std::runtime_error(
	"cannot append NULL to NOT NULL " + col->Type()->GetName() + " column"
	);