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.
The thing is, conceptually the node should use the default value if its path was not "present" in the source document. And a path is present if and only if applying it to the document yields a text result -- even an empty text result (which can happen btw: if you have an empty-string attribute that you're accessing via a `/foo/@attr` path). A `<value></value>` document, though, is identical to `<value/>` -- the "value" element does not contain a text node (I checked back with the XML spec). And XXPath reflects that properly -- resolving the path `/value` on the document gives the element, and calling `.text` on that returns `nil`. So the path is not present in that case, and thus the default value should be set if one was supplied.

I think that's the most straightforward semantics that you could have in this case, so we should use it.