Check that we do not crash if a parallelism group is explicitly set to
None. Permits usage of the following pattern.
[lit.common.cfg]
lit_config.parallelism_groups['my_group'] = None
if <condition>:
lit_config.parallelism_groups['my_group'] = 3
[project/lit.cfg]
config.parallelism_group = 'my_group'
Reviewers: rnk
Differential Revision: https://reviews.llvm.org/D58305
llvm-svn: 354912
