res_config_odbc/res_odbc: Fix handling of non-text columns updates with empty values.

res_config_odbc/res_odbc: Fix handling of non-text columns updates with empty values.

This patch fixes setting nullable integer columns to NULL instead of an empty
string, which fails for PostgreSQL, for example. The current code is supposed
to do so, but the check is broken. The patch also allows the first column in
the list to be a nullable integer.

This patch also adds a compatibility setting in res_odbc.conf,
allow_empty_string_in_nontext. It is enabled by default. It should be disabled
for database backends (such as PostgreSQL) that require NULL instead of an
empty string for Integer columns.

Review: https://reviewboard.asterisk.org/r/3375

(issue ASTERISK-23459)
Reported by: zvision
patches:
  res_config_odbc.diff uploaded by zvision (License 5755)
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Merged revisions 411399 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 411408 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@411512 65c4cc65-6c06-0410-ace0-fbb531ad65f3