fix: correctly handle FunctionExpressions 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:

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 548

• Total lines of source code: 9760

• Total classes: 419

• Average Cyclomatic Number per line of code: 0.37 / 2

• Average Source Lines of Code per method: 6

• Total tech debt: 2072.0 hours

• Found issues: 5 ⚠

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #1124 (eca690e) into master (a352607) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1124   +/-   ##
=======================================
  Coverage   85.19%   85.20%           
=======================================
  Files         342      342           
  Lines        7540     7541    +1     
=======================================
+ Hits         6424     6425    +1     
  Misses       1116     1116           

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

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