Fix issue when row contains an empty nested array

[dourvaris]

Hi, I'd appreciate a review on this as I'm not certain correct approach was used to solve it.

This PR fixes an issue where clickhouse driver is raising a NotImplemented error when all rows in result set are empty nested arrays causing the following exception:

    175         data = nested_column._read_data(
    176             nested_column_size, buf,
--> 177             nulls_map=nulls_map
    178         )
    179

/usr/local/lib/python3.7/site-packages/clickhouse_driver/columns/base.py in _read_data(self, n_items, buf, nulls_map)
     93
     94     def _read_data(self, n_items, buf, nulls_map=None):
---> 95         items = self.read_items(n_items, buf)
     96
     97         if self.after_read_items:

/usr/local/lib/python3.7/site-packages/clickhouse_driver/columns/base.py in read_items(self, n_items, buf)
    105
    106     def read_items(self, n_items, buf):
--> 107         raise NotImplementedError
    108
    109     def read_state_prefix(self, buf):

NotImplementedError:

After the fix the queries work and match output from clickhouse:

Clickhouse client

Debian-99-stretch-64-minimal :) CREATE TABLE test (id Int32, a Array(Array(Array(Int32)))) ENGINE MergeTree ORDER BY id;

CREATE TABLE test
(
    `id` Int32,
    `a` Array(Array(Array(Int32)))
)
ENGINE = MergeTree
ORDER BY id

Ok.

0 rows in set. Elapsed: 0.016 sec.

Debian-99-stretch-64-minimal :) INSERT INTO test VALUES
:-]     (1, []),
:-]     (2, [[]]),
:-]     (3, [[[]]]),
:-]     (4, [[[],[]]]),
:-]     (5, [[[],[]],[]]),
:-]     (6, [[[],[]],[[]]]),
:-]     (7, [[[],[]],[[],[]]]),
:-]     (8, [[[1],[2]],[[],[]]])
:-]     (9, [[[1,2],[2,3]],[[],[]]])
:-] ,;

INSERT INTO test VALUES

Ok.

9 rows in set. Elapsed: 0.005 sec.

Debian-99-stretch-64-minimal :) SELECT * FROM test;

SELECT *
FROM test

┌─id─┬─a───────────────────────┐
│  1 │ []                      │
│  2 │ [[]]                    │
│  3 │ [[[]]]                  │
│  4 │ [[[],[]]]               │
│  5 │ [[[],[]],[]]            │
│  6 │ [[[],[]],[[]]]          │
│  7 │ [[[],[]],[[],[]]]       │
│  8 │ [[[1],[2]],[[],[]]]     │
│  9 │ [[[1,2],[2,3]],[[],[]]] │
└────┴─────────────────────────┘

9 rows in set. Elapsed: 0.005 sec.
Debian-99-stretch-64-minimal :) SELECT * FROM test limit 2;

SELECT *
FROM test
LIMIT 2

┌─id─┬─a────┐
│  1 │ []   │
│  2 │ [[]] │
└────┴──────┘

2 rows in set. Elapsed: 0.004 sec.

Python client

In [3]: clickhouse_driver.Client(...).execute('select * from test')
Out[3]: 
[(1, []),
 (2, [[]]),
 (3, [[[]]]),
 (4, [[[], []]]),
 (5, [[[], []], []]),
 (6, [[[], []], [[]]]),
 (7, [[[], []], [[], []]]),
 (8, [[[1], [2]], [[], []]]),
 (9, [[[1, 2], [2, 3]], [[], []]])]

In [4]: clickhouse_driver.Client(...).execute('select * from test limit 2') # this query fails before
Out[4]: [(1, []), (2, [[]])]``` 

[coveralls]

Coverage decreased (-0.03%) to 96.533% when pulling 503bbc4 on dourvaris:fix/empty-nested-array into d80c330 on mymarilyn:master.

[xzkostyan]

PR looks good to me.

Thanks.