#688: guard against nil format string in Sprintf#evaluate

[SAY-5]

Closes #688.

Factbase::Sprintf#evaluate read its first operand (the format string) with a bare [0], while every sibling operand already used the safe &.first. _values returns bare nil when the referenced property is absent from the fact, so (sprintf $missing_prop 'hi') raised NoMethodError: undefined method '[]' for nil, which the term dispatcher then reported as the misleading "Probably the term 'sprintf' is not defined".

This guards the format-string operand and returns nil for a missing property, matching how Env#evaluate was fixed in #666 for the same bug class (#655). Added a regression test.

[0crat]

@SAY-5 It is not a good idea to name Git branches the way you named this one: "fix-sprintf-nil-format". You've earned -6 points. Next time, better give your branch the same name as the number of the ticket that you are solving. In this case, a perfect name, for example, would be "688". Your running score is +18; don't forget to check your Zerocracy account too).