Fix error when editing path for shebang

[yous]

I was making a new script, and during writing the path for shebang, saw this error:

Error detected while processing CursorHold Autocommands for "<buffer=1>"..function polyglot#shebang#Detect[1]..<SNR>78_Filetype:
line   15:
E684: list index out of range: -1

It's because split(l:path, "/") returns an empty list, so this PR fixes it.

This can be reproduced by making a new file and insert the content of the following:

#!/

Then go to NORMAL mode or insert a space to see the error message.

[sheerun]

huh, thank you