No way to filter by emptiness; e.g. include only rows that have a value in a specified field

[mjwestgate]

A common complaint is that galah returns records that have empty values for specified fields. For example:

galah_call() |>
  galah_filter(year >= 2015) |>
  galah_select(species, basisOfRecord) |>
  atlas_occurrences()

...might contain rows with no species name. According to this stack overflow post, the way to enforce that a field should be 'filled' is with a query like (field:*); but the equivalent call in galah is galah_filter(field == *), which fails because it's an invalid equation. A solution would be to hack the 'expected' behaviour of galah_filter so it does what we want, e.g.:

• galah_filter(species != "") should exclude rows that don't have a value for species via (species:*)
• galah_filter(species == "") should only include rows that lack a species name, via (*:* AND -species:*)