Support NULL in dir parameter in mj_addFileVFS.

Fixes #2839.

PiperOrigin-RevId: 807208005
Change-Id: I8def3b1e388dc19fa2fa8a92d53abef30dee5b88

src/user/user_vfs.cc

@@ -219,7 +219,8 @@ int mj_addFileVFS(mjVFS* vfs, const char* directory, const char* filename) {
   VFS* cvfs = GetVFSImpl(vfs);
 
   // make full name
-  FilePath fullname = FilePath(directory, filename);
+  const char* dir = directory != nullptr ? directory : "";
+  FilePath fullname = FilePath(dir, filename);
 
   // strip path
   FilePath newname = StripPath(filename);

test/user/user_vfs_test.cc

@@ -106,6 +106,18 @@ TEST_F(UserVfsTest, AddFileStripPath) {
   mj_deleteVFS(&vfs);
 }
 
+TEST_F(UserVfsTest, NullDirectory) {
+  mjVFS vfs;
+  mj_defaultVFS(&vfs);
+
+  const std::string file = "engine/testdata/actuation/activation.xml";
+  const std::string path = GetTestDataFilePath(file);
+  mj_addFileVFS(&vfs, nullptr, path.c_str());
+
+  EXPECT_TRUE(HasFile(&vfs, "activation.xml"));
+  mj_deleteVFS(&vfs);
+}
+
 TEST_F(UserVfsTest, AddFileRepeat) {
   mjVFS vfs;
   mj_defaultVFS(&vfs);