sat-encoder: Fix debug print of a possible NULL string.
sat-encoder: Fix debug print of a possible NULL string.

If the unsplit_word is NULL (which can happen if there are alternatives)
the debug printouts gets clobbered. Define macro N(s) which returns
"(null)" when s is NULL.

opencog/link-grammar,MadBomber/link-grammar,linas/link-grammar,MadBomber/link-grammar,opencog/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,MadBomber/link-grammar,ampli/link-grammar,linas/link-grammar,linas/link-grammar,MadBomber/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,MadBomber/link-grammar,linas/link-grammar,ampli/link-grammar,MadBomber/link-grammar,ampli/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,MadBomber/link-grammar,ampli/link-grammar,MadBomber/link-grammar