[lit] Allow setting parallelism groups to None

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