fix: partially handle normal completion function body for avoid-redundant-async

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

• Documentation update
• Bug fix
• New rule
• Changes an existing rule
• Add autofixing to a rule
• Add a CLI option
• Add something to the core
• Other, please explain:

#1147

[codecov]

Codecov Report

Merging #1148 (48229a1) into master (7f9e642) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 48229a1 differs from pull request most recent head d49d7a4. Consider uploading reports for the commit d49d7a4 to get more accurate results

@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
+ Coverage   85.53%   85.56%   +0.02%     
==========================================
  Files         350      350              
  Lines        7897     7909      +12     
==========================================
+ Hits         6755     6767      +12     
  Misses       1142     1142              

Impacted Files	Coverage Δ
...ules/rules_list/avoid_redundant_async/visitor.dart	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more