fix: fixed issue with type check in avoid-unnecessary-type-assertions

[vlkonoshenko]

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

[ ] Documentation update
[X] 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:

What changes did you make? (Give an overview)

Fix comparing type in a rule.

[codecov]

Codecov Report

Merging #555 (423aac7) into master (c871a40) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
- Coverage   85.29%   85.28%   -0.02%     
==========================================
  Files         220      220              
  Lines        4530     4525       -5     
==========================================
- Hits         3864     3859       -5     
  Misses        666      666              

Impacted Files	Coverage Δ
...rtions/avoid_unnecessary_type_assertions_rule.dart	100.00% <ø> (ø)
...ist/avoid_unnecessary_type_assertions/visitor.dart	100.00% <100.00%> (ø)

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