Fix tidy.brmsfit() output for model without a ran_pars component

[matthieu-bruneaux]

Fixes #147.

Changes to R/brms_tidiers.R:

• In the tidy.brmsfit() function, reformat the "group" and "term" columns of the out object in the case when nrow(res_list$fixed) > 0 but nrow(res_list$ran_pars) == 0 (in this case, those columns were lists with NULL values instead of vectors of NA).

Changes to inst/extdata/run_examples.R:

• Add a new example brms_brm_fit4 for the brms package, taken from ?brms::brm. This example is used to test that the fix to tidy.brmsfit() works fine.
• This added example is saved to inst/extdata/brms_example.rda with the other brms examples already present.

Changes to tests/testthat/test-brms.R:

• Add some basic tests to check that the columns "effect", "component", "group", and "term" of tidy.brmsfit() output are correct, using some brms examples already shipped with broom.mixed and the newly added brms example brms_brm_fit4.

[bbolker]

Sorry this slipped through the cracks!

[matthieu-bruneaux]

Thank you for accepting this PR @bbolker! And thanks a lot for the great package!