Python binding: Fix a StopIteration exception on no linkages
Python binding: Fix a StopIteration exception on no linkages

If a sentence has no linkages, then
	linkage = linkages.next()
raises StopIteration.
Instead of doing that, just return None.

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