Fixed NumericNode behavior with empty/nil text values

When a numeric node finds an empty value at its configured path, it should not throw if a default value had been provided.
Well, I have mixed feelings about this one... I see your point, but in the case of a numeric value, an empty string is as good as nil... it's not an "invalid value", it's a "no value", so that's why I had checked for nil *and* empty here.