Fix error when "hunk" does not exist in sy
Fix error when "hunk" does not exist in sy

Error

```console
Error detected while processing function sy#util#refresh_windows[9]..sy#start[51]..sy#stop[3]..sy#sign#remove_all_signs:
line    3:
E716: Key not present in Dictionary: hunks
```

Solution
Defensive programming. Ensure to have values for each required variables
in `sy#sign#remove_all_signs`.

mhinz/vim-signify