Fix crash on empty values

[m-rinaldi]

Fix crash on empty values

A setting with an empty value makes loadFile throw, so the application never starts.

Minimal example for reproducing it

.env:

OPTIONAL_SETTING=

Main.purs:

main = launchAff_ do
  Dotenv.loadFile
  liftEffect $ log "Loaded successfully."

Crashes with:

Error: Expected variable value

The same happens for OPTIONAL_SETTING="" and OPTIONAL_SETTING='', anywhere in the file.

Cause

The value parser requires at least one piece (some), so an empty value never matches.

Fix

Allows zero pieces (many) in quoted and unquoted values, and maps an empty value to LiteralValue "". Empty values now resolve to the empty string.

Also adds a parser test for A=.