AUC is calculated when only a single concentration is given

[billdenney]

This should be NA and not 0. It applies to all AUC calculations.

library(PKNCA)
#> 
#> Attaching package: 'PKNCA'
#> The following object is masked from 'package:stats':
#> 
#>     filter
pk.calc.auc.last(conc=1, time=0)
#> [1] 0

^{Created on 2022-08-26 by the reprex package (v2.0.1)}