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.
rather than just raising a generic runtime exception, we should raise NoAttrValueSet here, as that's the exception xml-mapping prescribes for cases like this -- i.e. when a node determines its path is not present in the XML. (the default_when_xpath_err thing raises the same exception if its block failed to find a path). The exception message would preferably also be similar to what default_when_xpath_err uses -- "Attribute #{@attrname} not set (XML text empty)" or something like that would do.