fix(pkg-r): DBI::SQL() errors given NULL

[cpsievert]

Introduced by #149. I'm pretty sure the DBI::SQL() isn't needed in the first place, so we can just drop it.

Here's a reprex:

library(shiny)
library(bslib)
library(querychat)
library(palmerpenguins)

qc <- QueryChat$new(penguins)

ui <- page_sidebar(
  sidebar = qc$sidebar(),
  dataTableOutput("table")
)

server <- function(input, output, session) {
  qc_vals <- qc$server()
  
  output$table <- renderDataTable({
    qc_vals$df()
  })
}

shinyApp(ui, server)

Warning: Error in initialize: cannot use object of class “NULL” in new():  class “SQL” does not extend that class

[cpsievert]

@gadenbuie merging to un-break main, but please comment if you'd like