[BUG] df_to_json generates empty json file

[chainsawriot]

Please confirm the following

• I have searched the existing issues
• The behaviour of the program is deviated from what is described in the documentation.
• I can reproduce this problem for more than one time.
• This is NOT a 3-digit error -- it does not display an error message like something went wrong. Status code: 400.
• This is a 3-digit error and I have consulted the Understanding API errors vignette and the suggestions do not help.

Describe the bug

ref #304

The internal function df_to_json generates empty data_.json when there is no data slot in the input df (BTW, this name is extremely confusing because df is actually a list, not a dataframe as the name implies).

Expected Behavior

When the input df doesn't have the data, it shouldn't write a json.

Steps To Reproduce

require(academictwitteR)
#> Loading required package: academictwitteR

tempdir <- academictwitteR:::.gen_random_dir()
academictwitteR:::create_data_dir(tempdir, verbose = FALSE)
whatever <- list()

academictwitteR:::df_to_json(df = whatever, data_path = tempdir)
list.files(tempdir)
#> [1] "data_.json"  "users_.json"

whatever$data <- iris
whatever$data$id <- 123
whatever$includes <- mtcars

academictwitteR:::df_to_json(df = whatever, data_path = tempdir)
list.files(tempdir)
#> [1] "data_.json"     "data_123.json"  "users_.json"    "users_123.json"

require(academictwitteR)
sessionInfo()
#> R version 4.1.2 (2021-11-01)
#> Platform: x86_64-pc-linux-gnu (64-bit)
#> Running under: Ubuntu 20.04.4 LTS
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/atlas/libblas.so.3.10.3
#> LAPACK: /usr/lib/x86_64-linux-gnu/atlas/liblapack.so.3.10.3
#> 
#> locale:
#>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=de_DE.UTF-8        LC_COLLATE=en_US.UTF-8    
#>  [5] LC_MONETARY=de_DE.UTF-8    LC_MESSAGES=en_US.UTF-8   
#>  [7] LC_PAPER=de_DE.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C       
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] academictwitteR_0.3.1
#> 
#> loaded via a namespace (and not attached):
#>  [1] knitr_1.37        magrittr_2.0.2    usethis_2.1.5     R.cache_0.15.0   
#>  [5] rlang_1.0.2       fastmap_1.1.0     fansi_1.0.2       stringr_1.4.0    
#>  [9] styler_1.6.2      highr_0.9         tools_4.1.2       xfun_0.29        
#> [13] R.oo_1.24.0       utf8_1.2.2        cli_3.2.0         withr_2.5.0      
#> [17] htmltools_0.5.2   ellipsis_0.3.2    yaml_2.3.5        digest_0.6.29    
#> [21] tibble_3.1.6      lifecycle_1.0.1   crayon_1.5.0      purrr_0.3.4      
#> [25] R.utils_2.11.0    vctrs_0.3.8       fs_1.5.2          glue_1.6.2       
#> [29] evaluate_0.14     rmarkdown_2.11    reprex_2.0.1      stringi_1.7.6    
#> [33] compiler_4.1.2    pillar_1.7.0      R.methodsS3_1.8.1 backports_1.4.1  
#> [37] jsonlite_1.8.0    pkgconfig_2.0.3

^{Created on 2022-03-10 by the reprex package (v2.0.1)}

Anything else?

No response