sqlp: fix read_csv optimization

the if condition to check if there were no sqlp options passed was faulty.

This caused `sqlp` to ALWAYS use the slow path, rather than using the fast path read_csv optimization when no options were passed

Also removed the deeplink to polars-sql source code and just point to Polars SQL reference manual. The manual is maintained and shows the Polars SQL syntax, even though its targeted to py-polars users.

src/cmd/sqlp.rs

@@ -6,11 +6,8 @@ Polars SQL is a SQL dialect, converting SQL queries to fast Polars LazyFrame exp
 (see https://docs.pola.rs/user-guide/sql/intro/).
 
 For a list of SQL functions and keywords supported by Polars SQL, see
-https://github.com/pola-rs/polars/blob/ee9bafbdef7d62baa06d469f42e6cec0755eb544/crates/polars-sql/src/functions.rs#L32 and
-https://github.com/pola-rs/polars/blob/ee9bafbdef7d62baa06d469f42e6cec0755eb544/crates/polars-sql/src/keywords.rs.
-https://docs.pola.rs/py-polars/html/reference/sql/index.html also provides a more readable
-version of the SQL functions and keywords, though be aware that it's for the Python version
-of Polars, so there will be some minor syntax differences.
+https://docs.pola.rs/py-polars/html/reference/sql/index.html though be aware that it's for
+the Python version of Polars, so there will be some minor syntax differences.
 
 Returns the shape of the query result (number of rows, number of columns) to stderr.
 
@@ -679,12 +676,12 @@ pub fn run(argv: &[&str]) -> CliResult<()> {
         && delim == b','
         && !args.flag_no_optimizations
         && !args.flag_try_parsedates
-        && args.flag_infer_len != 10_000
+        && args.flag_infer_len == 10_000 // make sure this matches the usage text default
         && !args.flag_streaming
         && !args.flag_low_memory
         && !args.flag_truncate_ragged_lines
         && !args.flag_ignore_errors
-        && args.flag_rnull_values.is_empty()
+        && rnull_values == vec![PlSmallStr::EMPTY]
         && !args.flag_decimal_comma
         && comment_char.is_none()
         && std::path::Path::new(&args.arg_input[0])

tests/test_sqlp.rs

@@ -1532,7 +1532,7 @@ fn sqlp_length_fns() {
     let expected = vec![
         svec!["words", "n_chrs1", "n_chrs2", "n_chrs3", "n_bytes", "n_bits"],
         svec!["Cafe", "4", "4", "4", "4", "32"],
-        svec!["", "", "", "", "", ""],
+        svec!["", "0", "0", "0", "0", "0"],
         svec!["東京", "2", "2", "2", "6", "48"],
     ];
 
@@ -1666,6 +1666,7 @@ fn sqlp_string_replace() {
     let expected = vec![
         svec!["words"],
         svec!["English breakfast tea is the best tea"],
+        svec![""],
     ];
 
     assert_eq!(got, expected);