Size checker fails when run against empty commits

[davidrusu]

https://github.com/maidsafe/sn_consensus/runs/6574526014?check_suite_focus=true

I created an empty commit using git commit --allow-empty to trigger a version bump and the size checker failed with exit code 1:

Run size=$(git diff --stat origin/master \
  size=$(git diff --stat origin/master \
  | grep -v .lock \
  | awk -F"|" '{ print $2 }' \
  | awk '{ print $1 }' \
  | sed '/^$/d' \
  | paste -sd+ - \
  | bc)
  
  echo "size=${size}" >> $GITHUB_ENV
  echo ""
  echo "Total lines changed (note: *.lock files are excluded from this count): "
  echo $size
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    CARGO_TERM_VERBOSE: true
    RUST_BACKTRACE: 1
    RUSTFLAGS: -D warnings
Error: Process completed with exit code 1.