A snapshot that contains join rules looks like empty.

[joohokim1]

Describe the bug
Join rule works fine in sampling mode, but when I created a snapshot, the result was empty.
As a result of test, it was not the problem of the snapshot detail page.
The dataset became empty, right after applying a join rule.

To Reproduce
Steps to reproduce the behavior:

1. Import & wrangle :
   auspol2019.small.txt
   location_geocode.csv.txt

2. Open a edit session with auspol2019 W.DS.

3. Apply a join rule with geocode W.DS; predicates are user_location, name each.

4. Generate any type of snapshots. file, hive both are empty.

Expected behavior
Pretty results that are same to samplings.

Screenshots

Desktop (please complete the following information):

• OS: MacOS
• Browser: Chrome
• Version: 68.0.3440.106

Additional context
The empty result was because of the parallel execution.
Accidentally, join has been run parallelly which we shouldn't for any type of aggregations for now.

But after fixing it, a NPE came out when matching the predicates.
I'm on it.

[joohokim1]

Integrated test #1 passed.