[FIX] orm field (new api): preserve empty values instead of folding to False

The new API introduced a small behavior change where empty
string values written or stored in a char/text field were
replaced by False (i.e. as if they were NULL).
This was done to mimic the web client behavior, but introduces
a very surprising effect: a.name = ""; assert a.name == "";
would fail. It would also require many more tests in the
code when reading existing required values from the database,
as they could still be False when an empty string value
had previously been stored, for some reason.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64