BNFC crashes on input that has no rules

[andreasabel]

$ bnfc --haskell -m -d Empty.cf 

0 rules accepted

Use Alex 3.0 to compile Empty/Lex.x.
Empty/Par.y Tested with Happy 1.15
writing new file ./Empty/Abs.hs
writing new file ./Empty/Lex.x
writing new file ./Empty/Par.y
writing new file ./Empty/Test.hs
bnfc: Prelude.head: empty list

[andreasabel]

BNFC now checks that there is at least one rule, exits otherwise.