[javascript mode] Be more robust about missing expressions
[javascript mode] Be more robust about missing expressions

Issue https://github.com/codemirror/CodeMirror/issues/6693

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