Fix invalid notebooks by keeping execution_count as null (Issue #19)

[adsepuri35]

Fixes Issue #19

When running nbstripout-fast with --keep-output and without --keep-count, the execution_count field was being removed. execution_count is required by nbformat, so we got invalid notebooks.

The PR sets execution_count to null instead of deleting it, keeping valid notebook structure.

Validation

cargo run -- --keep-output -t tests/test_notebook.ipynb > /tmp/stripped.ipynb

python3 - <<'EOF'
import nbformat
from nbformat.validator import validate
nb = nbformat.read('/tmp/stripped.ipynb', as_version=4)
validate(nb)
print("OK")
EOF

[peytondmurray]

Thank you for the contribution! Merged.