Generated json is invalid when tests run in parallel

[dsandi]

If mocha is executed with the --parallel flag, the generated mochawesome.json can't be used to generated an html report.

marge mochawesome-report/mochawesome.json

✘ Some files could not be processed:
mochawesome-report/mochawesome.json
  Invalid value null supplied to /results/0/suites/0/suites/0/suites/0/tests/0/pending: Boolean

When the tests are not run in parallel then the json is valid.

marge should be updated to support json files generated when running tests in parallel.