[mode/simple addon] Fix null dereference error on empty sub-matches
[mode/simple addon] Fix null dereference error on empty sub-matches

Closes #6618

wingify/CodeMirror,dperetti/CodeMirror,pabloferz/CodeMirror,wingify/CodeMirror,wingify/CodeMirror,nightwing/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,hackmdio/CodeMirror,nightwing/CodeMirror,pabloferz/CodeMirror,dperetti/CodeMirror,nightwing/CodeMirror,hackmdio/CodeMirror,pabloferz/CodeMirror