hotfix: npe in IndentationRule

[petertrr]

What's done:

• Fixed bug

This pull request closes #553

[codecov]

Codecov Report

Merging #595 (1872a98) into master (4ce391a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #595      +/-   ##
============================================
+ Coverage     80.82%   80.84%   +0.02%     
  Complexity     1704     1704              
============================================
  Files            87       87              
  Lines          4365     4365              
  Branches       1345     1346       +1     
============================================
+ Hits           3528     3529       +1     
  Misses          297      297              
+ Partials        540      539       -1     

Flag	Coverage Δ	Complexity Δ
unittests	80.84% <100.00%> (+0.02%)	0.00 <0.00> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
.../cqfn/diktat/ruleset/utils/indentation/Checkers.kt	78.21% <100.00%> (+0.99%)	0.00 <0.00> (ø)

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[aktsay6]

lgtm