Bug 1061758 - Suppress the job name from Mochitest-OOP symbols too

Treeherder equivalent of:
https://hg.mozilla.org/webtools/tbpl/rev/718d944e6511#l3.13
Bug 1062579 - Ensure get_symbol() cannot return an empty job symbol

In the case where 'n' wasn't found for jobs whose symbol letter was
suppressed (eg: a mochitest job that isn't split into multiple parts),
get_symbol() could have previously returned an empty symbol.
The suppression now only takes place if 'n' was found.