Literal values don't survive stat computation

[teunbrand]

I ran into this problem elsewhere, and I'm just posting this so we don't forget to fix this

VISUALISE bill_len AS x FROM ggsql:penguins
  DRAW histogram
    MAPPING 'foo' AS stroke
#> Failed to generate Vega-Lite output: Validation error: Column '__ggsql_aes_stroke__' referenced in aesthetic 'stroke' (layer 1 (global data)) does not exist.
#> Available columns: __ggsql_aes_x__, __ggsql_aes_x2__, __ggsql_aes_y__, __ggsql_aes_y2__