Rename MapGetWithNotNullAsserSpec to follow test convention

[cortinico]

Just found this test file that doesn't follow our naming convention. I'm renaming it.

[codecov]

Codecov Report

Merging #3878 (0bff47d) into main (5b2446d) will decrease coverage by 0.06%.
The diff coverage is 75.00%.

❗ Current head 0bff47d differs from pull request most recent head 9cfea8a. Consider uploading reports for the commit 9cfea8a to get more accurate results

@@             Coverage Diff              @@
##               main    #3878      +/-   ##
============================================
- Coverage     83.56%   83.50%   -0.07%     
- Complexity     3114     3115       +1     
============================================
  Files           456      456              
  Lines          8974     9001      +27     
  Branches       1751     1757       +6     
============================================
+ Hits           7499     7516      +17     
- Misses          561      566       +5     
- Partials        914      919       +5     

Impacted Files	Coverage Δ
...rbosch/detekt/rules/style/optional/OptionalUnit.kt	80.85% <50.00%> (-1.38%)	⬇️
...urbosch/detekt/generator/collection/RuleVisitor.kt	89.88% <80.00%> (-0.23%)	⬇️
...arturbosch/detekt/generator/collection/KDocTags.kt	67.50% <100.00%> (-14.56%)	⬇️
...n/kotlin/io/github/detekt/report/html/HtmlUtils.kt	98.00% <0.00%> (-2.00%)	⬇️
.../io/github/detekt/parser/KotlinEnvironmentUtils.kt	75.00% <0.00%> (+1.19%)	⬆️
...urbosch/detekt/rules/performance/SpreadOperator.kt	78.78% <0.00%> (+2.12%)	⬆️

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