[clang-format] Fix misaligned trailing comments in the presence of an empty block comment.

Fixes https://github.com/llvm/llvm-project/issues/53441.

Expected code:
```
/**/   //
int a; //
```

was before misformatted to:
```
/**/     //
int a; //
```

Because the "remaining length" (after the starting `/*`) of an empty block comment `/**/` was computed to be 0 instead of 2.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D118475