STR-312: Implement failure by returning NULL (instead of setjmp/longjmp) 
STR-312: Implement failure by returning NULL (instead of setjmp/longjmp) 

- don't cache empty list
- cast cached lists when used in ATinsert


svn path=/trunk/StrategoXT/; revision=12519

Apanatshka/strategoxt,lichtemo/strategoxt,Apanatshka/strategoxt,metaborg/strategoxt,metaborg/strategoxt,lichtemo/strategoxt,metaborg/strategoxt,lichtemo/strategoxt,Apanatshka/strategoxt,metaborg/strategoxt,Apanatshka/strategoxt,Apanatshka/strategoxt,lichtemo/strategoxt,lichtemo/strategoxt,metaborg/strategoxt