Fix --nozip_undeclared_test_outputs on Windows

Without this change, testing with `--nozip_undeclared_test_outputs`
always fails with this error:

```
FATAL: MappedOutputFile(): CreateFileW() failed: (error: 3): The system cannot find the path specified.
```

src/test/py/bazel/bazel_windows_test.py

@@ -397,6 +397,46 @@ def testRunWithScriptPath(self):
     self.AssertExitCode(exit_code, 0, stderr)
     self.assertIn('Hello from test!', '\n'.join(stdout))
 
+  def testZipUndeclaredTestOutputs(self):
+    self.CreateWorkspaceWithDefaultRepos('WORKSPACE')
+    self.ScratchFile('BUILD', [
+        'sh_test(',
+        '  name = "foo_test",',
+        '  srcs = ["foo.sh"],',
+        ')',
+        '',
+    ])
+    self.ScratchFile('foo.sh', [
+        'touch "$TEST_UNDECLARED_OUTPUTS_DIR/foo.txt"',
+    ])
+
+    exit_code, stdout, stderr = self.RunBazel(['info', 'bazel-testlogs'])
+    self.AssertExitCode(exit_code, 0, stderr)
+    bazel_testlogs = stdout[0]
+
+    output_file = os.path.join(bazel_testlogs, 'foo_test/test.outputs/foo.txt')
+    output_zip = os.path.join(bazel_testlogs,
+                              'foo_test/test.outputs/outputs.zip')
+
+    # Run the test with undeclared outputs zipping.
+    exit_code, _, stderr = self.RunBazel([
+        'test',
+        '--zip_undeclared_test_outputs',
+        '//:foo_test',
+    ], )
+    self.AssertExitCode(exit_code, 0, stderr)
+    self.assertFalse(os.path.exists(output_file))
+    self.assertTrue(os.path.exists(output_zip))
+
+    # Run the test without undeclared outputs zipping.
+    exit_code, _, stderr = self.RunBazel([
+        'test',
+        '--nozip_undeclared_test_outputs',
+        '//:foo_test',
+    ], )
+    self.AssertExitCode(exit_code, 0, stderr)
+    self.assertTrue(os.path.exists(output_file))
+    self.assertFalse(os.path.exists(output_zip))
 
 if __name__ == '__main__':
   unittest.main()

tools/test/windows/tw.cc

@@ -1298,8 +1298,10 @@ bool StartSubprocess(const Path& path, const std::wstring& args,
 }
 
 bool ArchiveUndeclaredOutputs(const UndeclaredOutputs& undecl) {
-  if (undecl.root.Get().empty()) {
-    // TEST_UNDECLARED_OUTPUTS_DIR was undefined, there's nothing to archive.
+  if (undecl.root.Get().empty() || undecl.zip.Get().empty()) {
+    // TEST_UNDECLARED_OUTPUTS_DIR was undefined, so there's nothing to archive,
+    // or TEST_UNDECLARED_OUTPUTS_ZIP was undefined as
+    // --nozip_undeclared_test_outputs was specified.
     return true;
   }