NPE in COMPLEX_BOOLEAN_EXPRESSION rule

[kgevorkyan]

What's done

• Modify warning message in aim to avoid confusions
• Fix NPE for conditions, which simplified to single statements in fix mode - they have no additional parenthesis

[codecov]

Codecov Report

Merging #1052 (e0199e6) into master (56168af) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##             master    #1052      +/-   ##
============================================
- Coverage     83.41%   83.40%   -0.02%     
- Complexity     2446     2447       +1     
============================================
  Files           102      102              
  Lines          6120     6122       +2     
  Branches       1832     1833       +1     
============================================
+ Hits           5105     5106       +1     
  Misses          273      273              
- Partials        742      743       +1     

Flag	Coverage Δ
unittests	83.40% <80.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...t/ruleset/rules/chapter3/BooleanExpressionsRule.kt	92.45% <75.00%> (-0.82%)	⬇️
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	98.64% <100.00%> (ø)

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