Fix bash for loop with default option

nb-convert.sh

@@ -3,6 +3,7 @@
 set -e
 
 jupyter nbconvert --TagRemovePreprocessor.enabled=True --TagRemovePreprocessor.remove_cell_tags no-convert --to script docs/examples/*.ipynb &&
+shopt -s nullglob && # Prevents the loop from running if there are no .txt files
 for file in docs/examples/*.txt;
 do mv -- "$file" "${file%.txt}.py";
 done