validateRequired enhancements

[drejc]

1. config.validateRequired() does not check for empty values only nulls
2. the throw UnsetValueException holds only item name, does not telling which exact item is not set.

For instance if you have a configuration like this:

host:
   url:
   port:

database:
   url:
   port:

UnsetValueException will for instance throw "item url not set", better would be "host.url not set"

[stale]

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