Bugfix. Useless auto-fix in AVOID_NULL_CHECKS rule

[aktsay6]

What's done:

• Fixed bug
• Added test

This pull request closes #840

[codecov]

Codecov Report

Merging #848 (04a431b) into master (c04b83f) will increase coverage by 0.04%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #848      +/-   ##
============================================
+ Coverage     81.00%   81.05%   +0.04%     
- Complexity     2282     2284       +2     
============================================
  Files           100      100              
  Lines          5843     5847       +4     
  Branches       1814     1813       -1     
============================================
+ Hits           4733     4739       +6     
  Misses          286      286              
+ Partials        824      822       -2     

Flag	Coverage Δ	Complexity Δ
unittests	81.05% <83.33%> (+0.04%)	2284.00 <0.00> (+2.00)

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Impacted Files	Coverage Δ	Complexity Δ
...fn/diktat/ruleset/rules/chapter4/NullChecksRule.kt	78.26% <83.33%> (+3.26%)	39.00 <0.00> (+2.00)

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[petertrr]

We can add the same logic for the opposite situation as well:

fun foo(y: Int?) {
    val x = if (y == null) {
        null
    } else {
        y + 2
    }
}

we shouldn't have useless run { null } block