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. At the moment, having falsy patterns in a pattern leads to _all_ triples being returned, which is quite problematic.

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.