Handle situation with 0 rows

[elong0527]

Seems we have bug when a PARAM has 0 rows after we adding more parameters using change in #18

Example code

meta <- meta_boxly(
  boxly_adsl,
  boxly_adlb,
  population_term = "apat",
  observation_term = "wk12",
  observation_subset = ANL01FL == "Y" & AVISITN <= 12 & !is.na(CHG)
  )

outdata <- prepare_boxly(meta,
  population = "apat",
  observation = "wk12",
  parameter = meta$plan$parameter,
  analysis = "boxly"
)

Message

Warning: In observation level data, the facet variable 'PARAM' is automatically transformed into a factor!
Warning: In observation level data, the group variable 'AVISITN' is automatically transformed into a factor!
Error in `$<-.data.frame`(`*tmp*`, outlier, value = NA) : 
  replacement has 1 row, data has 0