Omit line number for toml parse error for unknown spans

In the case that the span is (0..0), we don't usually know where the
error really occurred. In that case just omit the line number.

Resolves: #2189

This error is tracked upstream:

* toml: toml-rs/toml#589
* serde: serde-rs/serde#1183

src/util.rs

@@ -99,11 +99,15 @@ where
         let location_msg = err
             .span()
             .map(|span| {
-                let line = 1 + contents.as_bytes()[..(span.start)]
-                    .iter()
-                    .filter(|b| **b == b'\n')
-                    .count();
-                format!(" at line {line}")
+                if span == (0..0) {
+                    String::new()
+                } else {
+                    let line = 1 + contents.as_bytes()[..(span.start)]
+                        .iter()
+                        .filter(|b| **b == b'\n')
+                        .count();
+                    format!(" at line {line}")
+                }
             })
             .unwrap_or_default();
         Error::new(format!(