Add Design.empty

[xerial]

Resolves #395

[codecov]

Codecov Report

Merging #398 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
- Coverage   82.78%   82.77%   -0.01%     
==========================================
  Files         161      161              
  Lines        5942     5945       +3     
  Branches      589      591       +2     
==========================================
+ Hits         4919     4921       +2     
- Misses       1023     1024       +1

Impacted Files	Coverage Δ
...frame/src/main/scala/wvlet/airframe/Airframe.scala	100% <100%> (ø)	⬆️
...irframe/src/main/scala/wvlet/airframe/Design.scala	90% <100%> (+0.25%)	⬆️
...rframe/src/main/scala/wvlet/airframe/package.scala	91.66% <100%> (ø)	⬆️
.../main/scala/wvlet/airframe/launcher/Launcher.scala	91.86% <0%> (-0.7%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4edb2fa...15c3cba. Read the comment docs.