Fixing value of sys_candidate feature when only one duckling entity is found

[vembar]

For queries that have only one numeric entity identified by duckling, we end up setting the value of sys_candidate|type:{}|granularity:{}|pos:{} feature which indicates the number of sys_entity matches we had of a particular type and at a particular position to 0 (i.e, log(1)). This is problematic since DictVectorizer also fills in 0 for features not found in a query.

This PR ensures we have a non-zero value when this feature should be active for a query.
Here is some validation done with webex assistant. Context: before integrating the new mindmeld release candidate, the accuracy of the entity recognizer for dev and test sets of the select intent were 100 and 98.91 respectively.

With mindmeld version 4.3.5rc5

>>> er = nlp.domains['general'].intents['select'].entity_recognizer
>>> er.fit()
True
>>> er.evaluate(label_set='dev')
<EntityModelEvaluation score: 97.65%, 83 of 85 examples correct>
>>> er.evaluate(label_set='test')
<EntityModelEvaluation score: 97.83%, 90 of 92 examples correct>

With this change

>>> er = nlp.domains['general'].intents['select'].entity_recognizer
>>> er.fit()
True
>>> er.evaluate(label_set='dev')
<EntityModelEvaluation score: 100.00%, 85 of 85 examples correct>
>>> er.evaluate(label_set='test')
<EntityModelEvaluation score: 98.91%, 91 of 92 examples correct>
>>>