Move argument checks to helpers

[PietrH]

Functions repeat code regarding checking arguments, these could be moved into helpers to improve maintainability.

Example:

if (is.null(start_date)) {
    start_date_query <- "True"
  } else {
    start_date <- check_date_time(start_date, "start_date")
    start_date_query <- glue::glue_sql("log.datetime >= {start_date}", .con = connection)
  }

  # Check end_date
  if (is.null(end_date)) {
    end_date_query <- "True"
  } else {
    end_date <- check_date_time(end_date, "end_date")
    end_date_query <- glue::glue_sql("log.datetime < {end_date}", .con = connection)
  }

Not urgent.