Wordgraph display: Don't assert() on null-string subwords.
Wordgraph display: Don't assert() on null-string subwords.

Just display them as "(nothing)".
This is intended to make the program more robust on debugging,
especially when calling wordgraph_display() from the debugger.

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