tests/sitecustomize.py

memctx: substate needs to be {} instead of None

Setting substate to None was an oversight in 7cfd94ec5d30 and this patch
corrects it by setting substate to an empty dictionary which matches what
subrepo code expects.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass