empty kv dict can crash when dumping dict

[hendrikmuhs]

Fix a corner case when a kv dict is empty and being iterated over.

• initialize labels on position 0 and 1, to prevent lookup[1]
  • this works backwards compatible: old keyvi versions reading dicts from new version
• set start_state to 0 if dict is empty and check start_state !=0 for lookup
  • this works backwards compatible: new keyvi versions reading dicts from old version
  • shortcut lookup on empty dicts to prevent paging for empty dicts

fixes #360

[1] A transition is valid if labels[start_state + c] == c, we ran into this issue because for start_state = 1 the check looks like this: labels[1 + 0] == 0 - As labels are initialized with 0 - and because for empty dicts we write nothing, we had a transition that should not exist. This gets fixed by initializing labels[0](bonus) and labels[0] to something >1. For non-empty dicts construction takes care of setting labels[x], so no illegal state gets introduced.