Physics `\eval` with only the 2nd argument

The following renders correctly in LaTeX:

```latex
\pdv{x} x + 5 \eval_{y = 5}
```

With MathJax, it fails with:

> Missing argument for `\eval`

The workaround for MathJax is to add an empty `{}`:

```latex
\pdv{x} x + 5 \eval{}_{y = 5}
```

The workaround should not be necessary.