Treat falsy pattern components as unknowns.

Treating falsy components as unknowns is handier than only treating missing keys as unknowns, certainly in the case of programmatically created patterns.

For instance, [the Linked Data Fragments server now requires extra code to prepare the pattern](https://github.com/LinkedDataFragments/Server/commit/2e24bcddbe59aac05ba5886186551d378d3261eb#diff-83f98a6f71201fd121bdd58c18fd56fdR16).

This pull request treats falsy values in a pattern as unknowns.