[clang-format] unexpected formatting on empty lines

command `clang-format --lines=2:2 a.cc`

actual formatting:
```cpp
void f() {
  
}
```

expected formatting:
```cpp
void f() {}
```

this seems to be WAI if there are tokens on line 2, e.g:
```cpp
void f() {
  return;
}
```

gets formatted as `void f() { return; }`.