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When a "todo" exception is raised in a tree-sitter parser we tend to
dump the whole CST. These dumps can make the `stats.json` file very
large, which hangs the `-n` test:

    -  if [[ -e "$jsonfile" && -n "$(cat "$jsonfile")" ]]; then
    +  if [[ -s "$jsonfile" ]]; then

Helps #3538

test plan:
Go to https://app.circleci.com/pipelines/gh/returntocorp/semgrep?branch=parsing-stats
and check that the parsing-stats workflow now finishes in under 40
minutes instead of timing out, plus it archives the output of running
each language suite in a separate log file.