fix: Ensure empty sources have one "line"

src/renderer/source_map.rs

@@ -11,6 +11,21 @@ pub(crate) struct SourceMap<'a> {
 
 impl<'a> SourceMap<'a> {
     pub(crate) fn new(source: &'a str, line_start: usize) -> Self {
+        // Empty sources do have a "line", but it is empty, so we need to add
+        // a line with an empty string to the source map.
+        if source.is_empty() {
+            return Self {
+                lines: vec![LineInfo {
+                    line: "",
+                    line_index: line_start,
+                    start_byte: 0,
+                    end_byte: 0,
+                    end_line_size: 0,
+                }],
+                source,
+            };
+        }
+
         let mut current_index = 0;
 
         let mut mapping = vec![];

tests/formatter.rs

@@ -303,6 +303,7 @@ fn test_only_source() {
 error: 
  --> file.rs
   |
+1 |
 "#]];
     let renderer = Renderer::plain();
     assert_data_eq!(renderer.render(input), expected);

tests/rustc_tests.rs

@@ -2682,7 +2682,6 @@ LL | | */
 }
 
 #[test]
-#[should_panic(expected = "called `Option::unwrap()` on a `None` value")]
 fn mismatched_types1() {
     // tests/ui/include-macros/mismatched-types.rs