Bug 16848: Prevent invalid warning to be carped from output_pref

Bug 16848: Prevent invalid warning to be carped from output_pref

From Koha::DateUtils::output_pref:

   $dt = eval { dt_from_string( $str ) } if $str;
   carp "Invalid date '$str' passed to output_pref\n" if $@;

This second line is wrong: if $str does not exist, the first line is not evaluated and $@ could be filled with previous error.

To reproduce:

Then:
  prove t/DateUtils.t
will display:
t/DateUtils.t .. 20/60 Use of uninitialized value $str in concatenation (.) or string at Koha/DateUtils.pm line 217.
Invalid date '' passed to output_pref
 at t/DateUtils.t line 233.
t/DateUtils.t .. ok
All tests successful.
Files=1, Tests=60,  2 wallclock secs ( 0.02 usr  0.00 sys +  1.40 cusr  0.00 csys =  1.42 CPU)
Result: PASS

Test plan:
Without this patch, you should not see the carp

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>