Fix empty generation issue

[shubhamugare]

This issue resurfaced again with the last refactor.

Test:

python3 syncode/infer.py --mode grammar_strict --model google/gemma-2-2b-it --dataset json_eval --grammar json --max_new_tokens 400

The accuracy of JSON generation should be 99%. (This was reduced to 78% due to the bug)

• Added a unit test to ensure that empty generation is diallowed