FEAT: [Omnisci] fillna and nullif implementations

[semelianova]

Omnisci doesn't support ISNULL() function, but it is possible to replace it with another sql request.

[semelianova]

CI doesn't pass with 5.0.0 omnisci release, but test passes with the newest master. Non-empty LogicalValues are supported since commit, which isn't included in the latest release v5.1.0

[pep8speaks]

Hello @semelianova! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file ibis/tests/all/test_generic.py:

Line 31:21: F821 undefined name 'OmniSciDB'

Comment last updated at 2020-03-26 13:09:36 UTC

[anmyachev]

@semelianova use omnisci/core-os-cpu-dev:latest and fix pep8 errors

[anmyachev]

@semelianova should ibis/tests/all/test_generic.py::test_fillna_nullif pass since your change?

[semelianova]

@anmyachev Not completely, only 2 out of 4 cases which test fillna

[semelianova]

@xmnlab @jreback PR is ready for review

[xmnlab]

@semelianova LGTM. thanks for working on that.

just a comment, it is not a block for merging, but maybe if you change from _ifnull_workaround to _ifnull it would look better ... there are more operations that do a "workaround" but you don't need to add this to the function name.

could you add a release note for your changes please?

[xmnlab]

@jreback when you have time could you give a second review pls? thanks!

[xmnlab]

@semelianova I will ping here when #2084 is merged.

[jreback]

can you merge master, remove the updated image, and update the release note

[semelianova]

Need to wait for #2084 is merged

[anmyachev]

Need to wait for #2084 is merged

#2084 was be merged

[jreback]

can u merge master

checks are failing

[xmnlab]

@semelianova please try to use the omniscidb image tag used by master d7e34d4
the tag you set doesn't exist

[anmyachev]

@xmnlab this PR can be closed :)

[xmnlab]

thanks @semelianova @anmyachev !