Bug fix: units produce a crash with multispec data

[js3110]

Issue

Closes #925

Description

Multispec data was crashing due to AMOUNTU column having NA and non NA units, which had changes in processing after #907
This PR updates the units table build to allow NA units in AMOUNTU, as they are handled differently to AVALU and the others. AMOUNTU can have NA units if there is no VOLUME value.

Definition of Done

• App does not crash with multispec data

How to test

Try to run NCA with multispec data in testthat folder

Contributor checklist

• Code passes lintr checks
• Code passes all unit tests
• New logic covered by unit tests
• New logic is documented
• App or package changes are reflected in NEWS
• Package version is incremented
• R script works with the new implementation (if applicable)

[Gero1999]

Noticed the original error message mentioned unrelated parameters to AMOUNTU:

Warning: Error in pknca_unit_conversion: Units are provided for some but not all parameters; missing for: adj.r.squared, clast.pred, half.life, lambda.z, lambda.z.n.points, lambda.z.time.first, lambda.z.time.last, r.squared, span.ratio, tlast, tmax
This error can be converted to a warning using `PKNCA.options(allow_partial_missing_units = TRUE)`

But I guess this data-case solution made it work for the testing data... I will approve

[Gero1999]

For future reference, lets include the original issue that produced this: #907