Messages about `<pkg> is not installed` should only be shown if relevant to the data being converted

Here, the message is useless because no columns would be converted to `blob` anyway.

``` r
library(polars)
packageVersion("polars")
#> [1] '1.3.1'
rlang::is_installed("blob")
#> [1] FALSE

pl$LazyFrame(x = 1) |> 
  as.data.frame()
#> ℹ The `blob` package is not installed.
#> ℹ The blob class vector will not be printed correctly.
#>   x
#> 1 1
```



