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.
I'm not sure we should actually, it's gonna be a pain in the a** if we ever want to change the message at some stage, and I'm not sure error messages should be considered part of the public api? (which is to me the only reason we would consider testing their value)