collapse_rows does not work with col.names set to empty string

[yjml]

Describe the bug
collapse_rows does not collapse rows if the column name is set to empty string (via col.names in kable()). This used to work in <1.4 versions as I recall.

To Reproduce

library(knitr)
library(kableExtra)

df = structure(list(rn = c("n", "Age", "Age", "Measure", "Measure", "Cost", 
"Cost"), rn2 = c("n", "(mean (SD))", "(median [IQR])", 
"(mean (SD))", "(median [IQR])", "(mean (SD))", "(median [IQR])"
), Overall = c("    4090", "   60.65 (10.96)", "  59.00 [51.00, 65.00]", 
"    7.12 (1.48)", "   6.90 [6.20, 7.80]", "17411.75 (41009.85)", 
"4793.42 [2013.08, 14398.02]")), row.names = c(NA, -7L), class = "data.frame")

x <- knitr::kable(df, "html")
collapse_rows(x, 1L)

rn	rn2	Overall
n	n	4090
Age	(mean (SD))	60.65 (10.96)
	(median [IQR])	59.00 [51.00, 65.00]
Measure	(mean (SD))	7.12 (1.48)
	(median [IQR])	6.90 [6.20, 7.80]
Cost	(mean (SD))	17411.75 (41009.85)
	(median [IQR])	4793.42 [2013.08, 14398.02]

x1 <- knitr::kable(df, "html", col.names = c("", "", "Overall"))
collapse_rows(x1, 1L)

		Overall
n	n	4090
Age	(mean (SD))	60.65 (10.96)
Age	(median [IQR])	59.00 [51.00, 65.00]
Measure	(mean (SD))	7.12 (1.48)
Measure	(median [IQR])	6.90 [6.20, 7.80]
Cost	(mean (SD))	17411.75 (41009.85)
Cost	(median [IQR])	4793.42 [2013.08, 14398.02]

> sessionInfo()

R version 4.3.0 (2023-04-21)
Platform: x86_64-redhat-linux-gnu (64-bit)
Running under: Red Hat Enterprise Linux 8.9 (Ootpa)

Matrix products: default
BLAS:   /opt/R/4.3.0/lib64/R/lib/libRblas.so 
LAPACK: /opt/R/4.3.0/lib64/R/lib/libRlapack.so;  LAPACK version 3.11.0

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    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

time zone: Europe/Amsterdam
tzcode source: system (glibc)

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

other attached packages:
[1] kableExtra_1.4.0.3 knitr_1.45        

loaded via a namespace (and not attached):
 [1] svglite_2.1.3     cli_3.6.2         rlang_1.1.3       xfun_0.42         stringi_1.8.3     highr_0.10        jsonlite_1.8.8    glue_1.7.0        colorspace_2.1-0 
[10] htmltools_0.5.7   sass_0.4.8        scales_1.3.0      rmarkdown_2.25    evaluate_0.23     munsell_0.5.0     jquerylib_0.1.4   fastmap_1.1.1     lifecycle_1.0.4  
[19] stringr_1.5.1     compiler_4.3.0    rstudioapi_0.15.0 systemfonts_1.0.5 digest_0.6.34     viridisLite_0.4.2 R6_2.5.1          magrittr_2.0.3    bslib_0.6.1      
[28] tools_4.3.0       cachem_1.0.8      xml2_1.3.6       

[MMotaFerreira]

I've come across the same issue, in the stackoverflow they gave me a workaround. Use the unicode character for "NO-BREAK SPACE" - "\U00A0"

This works:

x2 <- knitr::kable(df, "html", col.names = c("\U00A0", "\U00A0", "Overall"))
collapse_rows(x2, 1L)

Here's the link for where I got the answer:
https://stackoverflow.com/questions/78326899/how-to-remove-the-name-of-first-column-of-a-table-with-collapse-rows-with-rmarkd/78327214