latexbuilder.rb: add nil check
latexbuilder.rb: add nil check

krororo/review,kuad9/review_study,orangain/review,murashitas/review,takahashim/review,krororo/review,krororo/review,kmuto/review,takahashim/review,suzuki/review,kuad9/review_study,murashitas/review,masawada/review,kmuto/review,orangain/review,masawada/review,kmuto/review,suzuki/review