fix(sbom): Don't set CARGO_SBOM_PATH when empty (like stable) (#16419)

### What does this PR try to resolve?

Noticed `CARGO_SBOM_PATH` was being set when playing around with
artifact-deps.

I could have gated the env by whether the nightly feature was enabled
but figured it would be better to not set if it there aren't SBOMs.

### How to test and review this PR?

Author: weihanglo

src/cargo/core/compiler/mod.rs

@@ -780,8 +780,11 @@ fn prepare_rustc(build_runner: &BuildRunner<'_, '_>, unit: &Unit) -> CargoResult
 
     if is_primary {
         base.env("CARGO_PRIMARY_PACKAGE", "1");
-        let file_list = std::env::join_paths(build_runner.sbom_output_files(unit)?)?;
-        base.env("CARGO_SBOM_PATH", file_list);
+        let file_list = build_runner.sbom_output_files(unit)?;
+        if !file_list.is_empty() {
+            let file_list = std::env::join_paths(file_list)?;
+            base.env("CARGO_SBOM_PATH", file_list);
+        }
     }
 
     if unit.target.is_test() || unit.target.is_bench() {

tests/testsuite/sbom.rs

@@ -22,18 +22,25 @@ fn append_sbom_suffix(link: &PathBuf) -> PathBuf {
 fn warn_without_passing_unstable_flag() {
     let p = project()
         .file("Cargo.toml", &basic_bin_manifest("foo"))
-        .file("src/main.rs", r#"fn main() {}"#)
+        .file(
+            "src/main.rs",
+            r#"fn main() {
+    eprintln!("{:?}", option_env!("CARGO_SBOM_PATH"));
+}"#,
+        )
         .build();
 
-    p.cargo("build")
+    p.cargo("run")
         .env("CARGO_BUILD_SBOM", "true")
         .masquerade_as_nightly_cargo(&["sbom"])
-        .with_stderr_data(
-            "\
-            [WARNING] ignoring 'sbom' config, pass `-Zsbom` to enable it\n\
-            [COMPILING] foo v0.5.0 ([..])\n\
-            [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [..]\n",
-        )
+        .with_stderr_data(snapbox::str![[r#"
+[WARNING] ignoring 'sbom' config, pass `-Zsbom` to enable it
+[COMPILING] foo v0.5.0 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[RUNNING] `target/debug/foo[EXE]`
+None
+
+"#]])
         .run();
 
     let file = append_sbom_suffix(&p.bin("foo"));