Bug 24640: Allow quotes.timestamp to be NULL

Bug 24640: Allow quotes.timestamp to be NULL

It is broken with strict mode enable, timestamp is created without
default value.

Note that the .sql installer file init the value with NOW(), which is
wrong.

DBD::mysql::st execute failed: Field 'timestamp' doesn't have a default
value [for Statement "INSERT INTO quotes (source, text) VALUES (?, ?);"
with ParamValues: 0="poeut", 1="pouet"] at
/kohadevbox/koha/tools/quotes/quotes_ajax.pl line 49.

Test plan:
Create a new quote of the day
=> Without this patch you will get an error

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>