Fix php warnings on some systems

Commit a0ae103:
When starting the timer, sometimes an array is used in mysql_real_escape_string, 
which results in empty string and a warning:
"Warning: mysql_real_escape_string() expects parameter 1 to be string, array given in /var/www/zeiten_multiple/live/core/libraries/mysql.class.php on line 1579‏"
Nothing dangerous. Also I wasn't able to reproduce the warning, therefore I don't know why it gets an array there - so I applied a hotfix, resulting in the same result without the warning. Added a TODO comment.

Commit 2737f23:
This is slightly more important, since warnings in the ajax response kills the handling of them.
When using certain settings some values aren't used, therefore they generate a warning because the variable doesn't exist. I added various isset()-querys.