OpenApi: Eliminate DefinitionGenerator#putIfNonNull(ObjectNode, String, Number)

We can eliminate DefinitionGenerator#putIfNonNull(ObjectNode, String, Number) method because:

1. range type is always Integer
2. com.google.common.collect.Range is designed to reject null values
3. key is never null - we have passed it into the method

Please note that this method is not the same as in NETCONF-1080.