qgis_sanitize_arguments(): fix loss of special arguments

[florisvdh]

This tries to solve #75. Because in qgis_sanitize_arguments(), the step that matches user arguments against algorithm arguments drops the special ELLIPSOID and PROJECT_PATH arguments, the latter were not passed to the qgis_process command.

@ThierryO can you give this a try?

[florisvdh]

Reasoning of 7d98d41:

named_empty_list <- rlang::set_names(list(), character())
named_empty_list
#> named list()
c(named_empty_list, named_empty_list)
#> list()

^{Created on 2022-01-21 by the reprex package (v2.0.1)}

[paleolimbot]

Thank you!

If it's easy to add a test for this (test_that("qgis_sanitize_arguments() doesn't drop special arguments", { ... }), could you do that here?

[florisvdh]

Tried that with 0b86162!

[paleolimbot]

This looks great! Thank you!