empty filter check

[yjunechoe]

Fixes #200

oa_query() now errors if it receives a filter with a missing value, instead of silently ignoring it. Guards against users accidentally specifying a filter that ends up not doing anything.

author_id <- NULL
author_works <- oa_fetch(
  entity = "works",
  author.id = author_id,
  publication_year = 2022,
  verbose = TRUE
)
#> Error: Filters must have a value: author.id

[trangdata]

Thank you for this PR! I made a small suggestion.