Fix snowball

[trangdata]

oa_snowball no longer errors out when a work hasn't been cited.

library(openalexR)

snowball_docs <- oa_snowball(
  identifier = c("W4295757800", "W4296128995", "W4297497355"),
  endpoint = "https://api.openalex.org/",
  verbose = FALSE
)
#> the list does not contain a valid OpenAlex collection

head(snowball_docs)
#> # A tibble: 6 × 27
#>   id        displ…¹ author ab    publi…² relev…³ so    so_id publi…⁴ issn  url  
#>   <chr>     <chr>   <list> <chr> <chr>   <lgl>   <chr> <chr> <chr>   <lis> <chr>
#> 1 https://… Deep R… <df>   Deep… 2016-0… NA      2016… <NA>  IEEE    <lgl> http…
#> 2 https://… SWISS-… <df>   Homo… 2018-0… NA      Nucl… http… Oxford… <chr> http…
#> 3 https://… Molecu… <df>   Oral… 2002-0… NA      Jour… http… Americ… <chr> http…
#> 4 https://… Highly… <df>   Prot… 2021-0… NA      Natu… http… Spring… <chr> http…
#> 5 https://… Extend… <df>   Exte… 2010-0… NA      Jour… http… Americ… <chr> http…
#> 6 https://… ChEMBL… <df>   ChEM… 2012-0… NA      Nucl… http… Oxford… <chr> http…
#> # … with 16 more variables: first_page <chr>, last_page <chr>, volume <chr>,
#> #   issue <chr>, is_oa <lgl>, cited_by_count <int>, counts_by_year <list>,
#> #   publication_year <int>, cited_by_api_url <chr>, ids <list>, doi <chr>,
#> #   type <chr>, referenced_works <list>, related_works <list>, concepts <list>,
#> #   role <chr>, and abbreviated variable names ¹​display_name,
#> #   ²​publication_date, ³​relevance_score, ⁴​publisher

^{Created on 2022-09-30 with reprex v2.0.2}

[trangdata]

Ready for review @massimoaria