formatter_glue fails when no unnamed arguments

Occasionally I forget (i.e., when interactive) to set `log_formatter(formatter_json)`. When this happens, any log messages that use named arguments will be blank. For instance,

```r
log_info(quux="foo", a=pi)
# INFO [2023-11-18 19:50:26]
```

where the initial intent was to have this:

```r
log_formatter(formatter_json)
log_info(quux="foo", a=pi)
# INFO [2023-11-18 19:50:54] {"quux":"foo","a":3.1416}
```

I'm fine with the fact that part of the reason is that I forgot the formatter, but it would be nice if `formatter_glue` did not drop named arguments. I suggest the names can be dropped, and pasted together (with a space). Something like:

```r
log_formatter(formatter_glue)
log_info(quux="foo", a=pi)
# INFO [2023-11-18 19:52:12] foo 3.14159265358979
```

---

_Edit_ ... as I'm thinking about this, the purpose and layout of `glue` is rather simple, where unnamed objects are strings to format, and named objects are variables to use for substitution ... so I think the `paste`-ing above should only be done when none of `...` are unnamed ... or perhaps just assume the first or first character or similar.

Bottom line, having it completely drop the message is perhaps not good. Thoughts?