Handle empty highlight group name

In my environment, `hlexists("")` somehow evaluates to `1`.
So, when `g:glowshi_ft_selected_hl_link` is empty, it produces following error:

```
Error detected while processing function glowshi_ft#gs_f..<SNR>208_glowshi_ft..<SNR>208_choose_pos..glowshi_ft#highlight:
line    2:
E412: Not enough arguments: ":highlight link GlowshiFtSelected"
```

I'm not sure if this is the case in all environments, so please ignore this if you think something's wrong in my environment.