Empty language crashes alex with (Array.!): undefined array element

Alex does not seem to handle the empty language well.  MWE:
```
alex --version
echo ":- [] ;" > Test.x
alex Test.x
```
gives
```
Alex version 3.2.6, (c) 2003 Chris Dornan and Simon Marlow
alex: (Array.!): undefined array element
```
I think Alex should report a proper error, or find a way to handle the empty language (e.g. by ignoring it as it can never match anything).