Test all imputation methods for all concentrations are NA, all times are NA, all concentrations are 0

[billdenney]

Add to each imputation method in tests/testthat/test-impute.R:

1. All concentrations are NA
2. All times are NA
3. All concentrations are 0

[Gero1999]

@billdenney I can take over this issue as soon as you agree/review on PR #351. I need to know what is the expected behavior of the functions in each scenario.

Also I am not sure if there is a way for you to give me more rights over the repo so I can assign myself to issues and link branches