Validate selected inputs from data_extract before passing to data_merge to avoid
dplyr errors or unexpected results.
check_data_merge_selectors(selector_list)NULL if check is successful and shiny validate error otherwise.
(reactive)
output from data_extract_multiple_srv() or a reactive named list of
outputs from data_extract_srv().
When using a reactive named list, the names must be identical to the shiny
ids of the respective
data_extract_ui().