[BUGFIX] remove PHP warnings from EXT:solr

because of a lack of testing a PHP Warning is generated in
StatisticsWriter.php on line 100

this patch added the missing check and prevent the PHP Warning

Change-Id: I1aaf59669cf32817c59544e6c62e8287c6311a41
Resolves: #60906
Documentation: #60906
Releases: 3.0, 3.1
Reviewed-on: http://review.typo3.org/32102
Reviewed-by: Hans Höchtl <jhoechtl@gmail.com>
Tested-by: Hans Höchtl <jhoechtl@gmail.com>
Reviewed-by: Phuong Doan <phuong.doan@dkd.de>
Tested-by: Phuong Doan <phuong.doan@dkd.de>