`plot()` not anticipating subclasses

[vincenzocoia]

Describe the bug
Some plot() methods do not anticipate subclasses of sfc objects. This makes it difficult to write extensions to the sf package.

To Reproduce
Here's an example with a linestring.

library(sf)
#> Linking to GEOS 3.11.0, GDAL 3.5.3, PROJ 9.1.0; sf_use_s2() is TRUE

l <- st_linestring(matrix(c(0, 1, 0, 1), ncol = 2))
s <- st_sfc(l)
plot(s)

class(s) <- c("subclass", class(s))
class(s)
#> [1] "subclass"       "sfc_LINESTRING" "sfc"

Failure:

try(plot(s))

#> Error in eval(expr, envir, enclos) : 
#>   Not compatible with STRSXP: [type=NULL].

The error is not because of the call to plot_sf(), although that plot is empty; it's because of the call to st_is_empty(), which seems to not be expecting subclasses:

plot_sf(s)
try(st_is_empty(s))
#> Error in eval(expr, envir, enclos) : 
#>   Not compatible with STRSXP: [type=NULL].

^{Created on 2024-08-31 with reprex v2.1.0}

Additional context

sessionInfo()
#> R version 4.3.3 (2024-02-29)
#> Platform: aarch64-apple-darwin20 (64-bit)
#> Running under: macOS Monterey 12.6
#> 
#> Matrix products: default
#> BLAS:   /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/lib/libRblas.0.dylib 
#> LAPACK: /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/lib/libRlapack.dylib;  LAPACK version 3.11.0
#> 
#> locale:
#> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
#> 
#> time zone: America/Vancouver
#> tzcode source: internal
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> loaded via a namespace (and not attached):
#>  [1] styler_1.10.3     digest_0.6.36     fastmap_1.2.0     xfun_0.45        
#>  [5] magrittr_2.0.3    glue_1.7.0        R.utils_2.12.3    knitr_1.47       
#>  [9] htmltools_0.5.8.1 rmarkdown_2.27    lifecycle_1.0.4   cli_3.6.3        
#> [13] R.methodsS3_1.8.2 vctrs_0.6.5       reprex_2.1.0      withr_3.0.0      
#> [17] compiler_4.3.3    R.oo_1.26.0       R.cache_0.16.0    purrr_1.0.2      
#> [21] rstudioapi_0.16.0 tools_4.3.3       evaluate_0.24.0   yaml_2.3.8       
#> [25] rlang_1.1.4       fs_1.6.4
sf::sf_extSoftVersion()
#>           GEOS           GDAL         proj.4 GDAL_with_GEOS     USE_PROJ_H 
#>       "3.11.0"        "3.5.3"        "9.1.0"         "true"         "true" 
#>           PROJ 
#>        "9.1.0"
packageVersion("sf")
#> [1] '1.0.16'

^{Created on 2024-08-31 with reprex v2.1.0}