missing_argument_linter() incorrect for multiple missing arguments

```r
lint(text = "foo(,,)", linters = missing_argument_linter())
# <text>:1:5: style: [missing_argument_linter] Missing argument 1 in function call.
# foo(,,)
#     ^
# <text>:1:6: style: [missing_argument_linter] Missing argument 2 in function call.
# foo(,,)
#      ^
```

But there are three missing arguments.