TrigValTools: fix check_log.py when ignorePattern is empty

Hello,

!79528 (merged) has uncovered a buggy behaviour of the script used to parse trigger test log files. The script is given a list of patterns to ignore, and uses constructs like re.find(pattern_to_ignore,logfile_line) to decide whether a line should be ignored. When there is no pattern to ignore, due to the regex behaviour, re.find('',logfile_line) will return a match, whatever logfile_line is... As a workaround, when no pattern is passed, (?!) is used, which should never match anything.

Adding the urgent label because our log parsing capability is largely affected.

Tagging @lbeemste .

Cheers, Bertrand