Changing functionality for AVOID_NULL_CHECK

[orchestr7]

What's done:

1. Now it is not trigered on simple cases like val a = b ==null
2. added logic for requireNotNull

[codecov]

Codecov Report

Merging #564 (3da8aa6) into master (052bd9d) will decrease coverage by 0.88%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #564      +/-   ##
============================================
- Coverage     81.68%   80.80%   -0.89%     
- Complexity     1641     1709      +68     
============================================
  Files            79       87       +8     
  Lines          4139     4381     +242     
  Branches       1307     1349      +42     
============================================
+ Hits           3381     3540     +159     
- Misses          229      298      +69     
- Partials        529      543      +14     

Flag	Coverage Δ	Complexity Δ
unittests	80.80% <85.71%> (-0.89%)	0.00 <42.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...at-rules/src/main/kotlin/generated/WarningNames.kt	0.00% <ø> (ø)	0.00 <0.00> (ø)
...fn/diktat/ruleset/rules/kdoc/CommentsFormatting.kt	69.79% <ø> (-0.21%)	76.00 <0.00> (-1.00)
...lin/org/cqfn/diktat/ruleset/rules/PackageNaming.kt	93.82% <50.00%> (-1.11%)	32.00 <0.00> (ø)
...in/org/cqfn/diktat/ruleset/rules/NullChecksRule.kt	75.00% <62.50%> (-2.78%)	22.00 <4.00> (+4.00)	⬇️
.../cqfn/diktat/ruleset/rules/BlockStructureBraces.kt	83.92% <85.71%> (+0.11%)	53.00 <0.00> (ø)
...ktat/ruleset/rules/AvoidEmptyPrimaryConstructor.kt	90.00% <90.00%> (ø)	8.00 <8.00> (?)
...iktat/ruleset/rules/ImplicitBackingPropertyRule.kt	93.75% <93.75%> (ø)	23.00 <23.00> (?)
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	98.34% <100.00%> (+0.02%)	10.00 <0.00> (ø)
...cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt	97.64% <100.00%> (+0.38%)	9.00 <0.00> (+1.00)
...in/org/cqfn/diktat/ruleset/rules/FunctionLength.kt	84.21% <100.00%> (-0.79%)	9.00 <0.00> (-1.00)
... and 22 more

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[petertrr]

Text can be adjusted a little: like use 'requireNotNull(myVar)' instead of 'require (myVar != null)'.

[orchestr7]

getting myVar is too complex logic - I would not like to make the code more complex, but the second part is easy enough, added