AWS ALB: Ensure to treat provider.alb.authorizers as optional

[medikoo]

Regression introduced with #8275

[codecov-commenter]

Codecov Report

Merging #8295 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8295   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files         250      250           
  Lines        9391     9391           
=======================================
  Hits         8280     8280           
  Misses       1111     1111           

Impacted Files	Coverage Δ
...ins/aws/package/compile/events/alb/lib/validate.js	97.82% <100.00%> (ø)

---

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 7f47448...93117ad. Read the comment docs.