os: fix tmpdir() returning 'undefined\temp' on Windows

When process.env is cleared (e.g., in sandboxed environments),
os.tmpdir() would return 'undefined\temp' on Windows due to
string concatenation with undefined values.

This fix:
- Checks for valid environment variables before using them
- Falls back to getTempDir() when env vars are unavailable
- Adds test case to prevent regression

Reported-by: @rhysd
Fixes: #60582

@ry Could you please review this fix?

Author: ish1416

lib/os.js

@@ -179,15 +179,24 @@ platform[SymbolToPrimitive] = () => process.platform;
  */
 function tmpdir() {
   if (isWindows) {
-    const path = process.env.TEMP ||
-           process.env.TMP ||
-           (process.env.SystemRoot || process.env.windir) + '\\temp';
+    const temp = process.env.TEMP || process.env.TMP;
+    if (temp) {
+      if (temp.length > 1 && temp[temp.length - 1] === '\\' && temp[temp.length - 2] !== ':') {
+        return StringPrototypeSlice(temp, 0, -1);
+      }
+      return temp;
+    }
 
-    if (path.length > 1 && path[path.length - 1] === '\\' && path[path.length - 2] !== ':') {
-      return StringPrototypeSlice(path, 0, -1);
+    const systemRoot = process.env.SystemRoot || process.env.windir;
+    if (systemRoot) {
+      const path = systemRoot + '\\temp';
+      if (path.length > 1 && path[path.length - 1] === '\\' && path[path.length - 2] !== ':') {
+        return StringPrototypeSlice(path, 0, -1);
+      }
+      return path;
     }
 
-    return path;
+    return getTempDir() || 'C:\\temp';
   }
 
   return getTempDir() || '/tmp';

test/parallel/test-os.js

@@ -57,6 +57,15 @@ if (common.isWindows) {
   assert.strictEqual(os.tmpdir(), '\\');
   process.env.TEMP = 'C:\\';
   assert.strictEqual(os.tmpdir(), 'C:\\');
+  
+  // Test case for issue #60582: tmpdir() should not return 'undefined\temp'
+  // when all environment variables are cleared
+  const originalEnv = { ...process.env };
+  process.env = {};
+  const result = os.tmpdir();
+  assert.ok(!result.includes('undefined'), 
+    `tmpdir() should not contain 'undefined', got: ${result}`);
+  process.env = originalEnv;
 } else {
   assert.strictEqual(os.tmpdir(), '/tmpdir');
   process.env.TMPDIR = '';