Turbopack: omit empty source map when code starts with a new line (#77734)

### What?

SourceMaps contain some unnecessary sections than can be avoided.

Author: sokra

test/e2e/app-dir/scss/compilation-and-prefixing/compilation-and-prefixing.test.ts

@@ -106,18 +106,6 @@ describe.each([
                       "line": 1,
                     },
                   },
-                  {
-                    "map": {
-                      "mappings": "A",
-                      "names": [],
-                      "sources": [],
-                      "version": 3,
-                    },
-                    "offset": {
-                      "column": 91,
-                      "line": 1,
-                    },
-                  },
                 ],
                 "version": 3,
               }
@@ -143,18 +131,6 @@ describe.each([
                       "line": 1,
                     },
                   },
-                  {
-                    "map": {
-                      "mappings": "A",
-                      "names": [],
-                      "sources": [],
-                      "version": 3,
-                    },
-                    "offset": {
-                      "column": 91,
-                      "line": 1,
-                    },
-                  },
                 ],
                 "version": 3,
               }

test/integration/css/test/css-compilation.test.js

@@ -158,18 +158,6 @@ module.exports = {
                           "line": 1,
                         },
                       },
-                      {
-                        "map": {
-                          "mappings": "A",
-                          "names": [],
-                          "sources": [],
-                          "version": 3,
-                        },
-                        "offset": {
-                          "column": 264,
-                          "line": 1,
-                        },
-                      },
                     ],
                     "version": 3,
                   }
@@ -216,18 +204,6 @@ module.exports = {
                           "line": 1,
                         },
                       },
-                      {
-                        "map": {
-                          "mappings": "A",
-                          "names": [],
-                          "sources": [],
-                          "version": 3,
-                        },
-                        "offset": {
-                          "column": 264,
-                          "line": 1,
-                        },
-                      },
                     ],
                     "version": 3,
                   }

turbopack/crates/turbopack-core/src/code_builder.rs

@@ -187,8 +187,13 @@ impl Code {
             }
             last_byte_pos = *byte_pos;
 
-            if pos.column != 0 || map.is_some() {
-                sections.push((pos, map.clone().unwrap_or_else(SourceMap::empty_rope)))
+            if let Some(map) = map {
+                sections.push((pos, map.clone()))
+            } else {
+                // We don't need an empty source map when column is 0 or the next char is a newline.
+                if pos.column != 0 && read.fill_buf()?.first().is_some_and(|&b| b != b'\n') {
+                    sections.push((pos, SourceMap::empty_rope()));
+                }
             }
         }