Fix ordered_by_ancestry for oracle-db

[lual]

An empty string is treated as NULL in Oracle.
COALESCE(NULL,'') is treated as NULL.
So Ordering ascending, deliver NULL-Values at the rear.

[coveralls]

Coverage decreased (-0.2%) to 97.837% when pulling 0483b1b on rtiss:ordered_by_ancestry_for_oracle into 2ff497e on stefankroes:master.

[coveralls]

Coverage decreased (-0.2%) to 97.831% when pulling 0483b1b on rtiss:ordered_by_ancestry_for_oracle into 2ff497e on stefankroes:master.

[coveralls]

Coverage decreased (-0.2%) to 97.831% when pulling 0483b1b on rtiss:ordered_by_ancestry_for_oracle into 2ff497e on stefankroes:master.

[kbrock]

@lual could you run with the patch from #541 and verify it meets your needs?

I'm pretty sure the postgresql solution will meet your needs. Wrapping in the ascend allows reverse ordering not to blow up.

[kbrock]

Better yet, would you make the changes I made over there and put them into your pr? that way you could get credit

[kbrock]

I'll give you a few days, otherwise I'll just merge that other PR and hopefully that will meet your needs

[kbrock]

leaving this one open until I share a good monkey patch to you.
the suggested code here would not pass the test suite, so I prefer/merged the other version
specifically when people reverse the order of the ancestry column

[kbrock]

monkey patch in #535 should hold you over until the next version of rails is released