linkage_testfile(): Prevent an exception if no linkages
linkage_testfile(): Prevent an exception if no linkages

If a sentence in the file has no linkages (it can happen due to library
bug) then the tests suite aborts on StopIteration without completing
rest of tests.

This is already prevented in case there are not enough linkages.
So fix it in a similar way.

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