cpp ATTENTION: Changed semantics for Payload entry casts to bool

For string values, ONLY the empty string "" maps to false
(previously, "false", "False", and "0" also mapped to false)
For list and map values, zero-element ones now map to false
(previously, any list or map would map to true).

The new semantics are identical to Python behavior for your convenience,
since they were not generically prescribed by the STL anyway.