[Bug]: count_abnormal_by_worst_grade returns error when there's only 1 unique PARAM

[JiaLiu0001]

What happened?

A bug happened!

1. count_abnormal_by_worst_grade returns error when there's only 1 unique PARAM. TLGC LBT07 is referred in the following script.
   Error: df[[variables$param]] is not a valid factor, please check the factor levels (no empty strings allowed)

2. However, if the PARAM contains more than 1 unique param, no error.

library(formatters)
library(stringr)
library(dplyr)
library(tern)
library(scda)
library(forcats)

adsl <- synthetic_cdisc_data("rcd_2022_02_28")$adsl %>%
  df_explicit_na()

adlb <- synthetic_cdisc_data("rcd_2022_02_28")$adlb %>%
  semi_join(adsl, by = "USUBJID") %>%
  select(USUBJID,TRT01A,PARAM,ATOXGR,ANRIND)

# Derive GRADE_DIR and GRADE_ANL to use in layout from ATOXGR
adlb_f <- adlb %>%
  mutate(
    GRADE_DIR = factor(case_when(
      ATOXGR %in% c("-1", "-2", "-3", "-4") ~ "LOW",
      ATOXGR == "0" ~ "ZERO",
      ATOXGR %in% c("1", "2", "3", "4") ~ "HIGH"),
      levels = c("LOW", "ZERO", "HIGH")),
    GRADE_ANL = fct_relevel(
      fct_recode(ATOXGR, `1` = "-1", `2` = "-2", `3` = "-3", `4` = "-4"), c("0", "1", "2", "3", "4")
    )
  ) %>%
  var_relabel(
    GRADE_DIR = "Direction of Abnormality",
    GRADE_ANL = "Analysis Grade"
  )
adlb_f <- adlb_f %>% mutate(PARAM = 'fake')
# Construct analysis map
map <- unique(adlb_f[adlb_f$GRADE_DIR != "ZERO", c("PARAM", "GRADE_DIR", "GRADE_ANL")]) %>% 
  lapply(as.character) %>%
  as.data.frame() %>%
  arrange(PARAM, desc(GRADE_DIR), GRADE_ANL)


lyt <- basic_table() %>%
  split_cols_by("TRT01A") %>%
  add_colcounts() %>%
  split_rows_by(
    "PARAM",
    label_pos = "topleft",
    split_label = 'Laboratory Test'
  ) %>%
  summarize_num_patients(
    var = "USUBJID",
    required = "ATOXGR",
    .stats = "unique_count"
  ) %>%
  split_rows_by(
    "GRADE_DIR",
    label_pos = "topleft",
    # split_label = 'Direction of Abnormality',
    split_label = obj_label(adlb_f$GRADE_DIR),
    split_fun = trim_levels_to_map(map)
  ) %>%
  count_abnormal_by_worst_grade(
    var = "GRADE_ANL",
    variables = list(id = "USUBJID", param  = "PARAM", grade_dir = "GRADE_DIR")
  ) %>%
  append_topleft("    Highest DAIDS Grading")

result <- build_table(
  lyt = lyt,
  df = adlb_f,
  alt_counts_df = adsl
)
result

sessionInfo()

R version 4.0.4 (2021-02-15)
Platform: x86_64-redhat-linux-gnu (64-bit)
Running under: Red Hat Enterprise Linux 8.4 (Ootpa)

Matrix products: default
BLAS:   /usr/lib64/libblas.so.3.8.0
LAPACK: /opt/mango/R/4.0.4/lib/R/lib/libRlapack.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8   
 [6] LC_MESSAGES=en_US.UTF-8    LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] forcats_0.5.1  scda_0.1.2     tern_0.7.6     rtables_0.4.1  magrittr_2.0.1 dplyr_1.0.5    stringr_1.4.0 

loaded via a namespace (and not attached):
 [1] pillar_1.6.0        compiler_4.0.4      tools_4.0.4         packrat_0.5.0       digest_0.6.27       checkmate_2.0.0     lifecycle_1.0.0    
 [8] tibble_3.1.1        gtable_0.3.0        lattice_0.20-41     pkgconfig_2.0.3     rlang_0.4.10        Matrix_1.3-2        rstudioapi_0.13    
[15] cli_2.4.0           DBI_1.1.1           generics_0.1.0      vctrs_0.3.7         grid_4.0.4          tidyselect_1.1.0    glue_1.4.2         
[22] R6_2.5.0            fansi_0.4.2         survival_3.2-10     dfoptim_2020.10-1   ggplot2_3.3.3       purrr_0.3.4         tidyr_1.1.3        
[29] scales_1.1.1        backports_1.2.1     ellipsis_0.3.1      htmltools_0.5.1.1   splines_4.0.4       assertthat_0.2.1    colorspace_2.0-0   
[36] numDeriv_2016.8-1.1 utf8_1.2.1          optimx_2020-4.2     stringi_1.5.3       munsell_0.5.0       broom_0.7.6         crayon_1.4.1

Relevant log output

No response

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