Do not consider LLDB version to be valid if it is empty

When dry run is enabled, the command for finding LLDB version would succeed, but return an empty string. This was inadvertently enabling a code path that should only be executed when the LLDB is actually present and its version is valid. This commit makes sure that if the version is empty, LLDB will be considered not found.

src/bootstrap/src/core/build_steps/test.rs

@@ -1805,14 +1805,9 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
 
         let lldb_exe = builder.config.lldb.clone().unwrap_or_else(|| PathBuf::from("lldb"));
         let lldb_version = builder
-            .run(
-                BootstrapCommand::new(&lldb_exe)
-                    .capture()
-                    .allow_failure()
-                    .run_always()
-                    .arg("--version"),
-            )
-            .stdout_if_ok();
+            .run(BootstrapCommand::new(&lldb_exe).capture().allow_failure().arg("--version"))
+            .stdout_if_ok()
+            .and_then(|v| if v.trim().is_empty() { None } else { Some(v) });
         if let Some(ref vers) = lldb_version {
             cmd.arg("--lldb-version").arg(vers);
             let lldb_python_dir = builder