Fix NPE in KdocFormatting rule

[petertrr]

What's done:

• Updated logic
• Added tests

We were treating any blank line in KDoc tag's body as an error and were trying to remove it, while empty lines between tag's lines are valid.

TODO:

• example with trailing blank line in tag's body

This pull request closes #868

[codecov]

Codecov Report

Merging #876 (cf75a65) into master (a2f4056) will decrease coverage by 0.08%.
The diff coverage is 60.00%.

@@             Coverage Diff              @@
##             master     #876      +/-   ##
============================================
- Coverage     83.61%   83.53%   -0.09%     
- Complexity     2208     2210       +2     
============================================
  Files           101      101              
  Lines          5669     5677       +8     
  Branches       1614     1620       +6     
============================================
+ Hits           4740     4742       +2     
  Misses          256      256              
- Partials        673      679       +6     

Flag	Coverage Δ	Complexity Δ
unittests	83.53% <60.00%> (-0.09%)	2210.00 <4.00> (+2.00)	⬇️

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

Impacted Files	Coverage Δ	Complexity Δ
.../kotlin/org/cqfn/diktat/ruleset/utils/KdocUtils.kt	20.83% <20.00%> (-0.22%)	0.00 <0.00> (ø)
...tlin/org/cqfn/diktat/ruleset/utils/AstNodeUtils.kt	84.77% <60.00%> (-0.53%)	0.00 <0.00> (ø)
...ktat/ruleset/rules/chapter2/kdoc/KdocFormatting.kt	83.90% <63.63%> (-0.93%)	89.00 <4.00> (ø)
...n/diktat/ruleset/rules/chapter3/MagicNumberRule.kt	86.04% <100.00%> (+0.68%)	27.00 <0.00> (+1.00)
.../ruleset/rules/chapter2/kdoc/CommentsFormatting.kt	71.81% <0.00%> (+0.67%)	77.00% <0.00%> (+1.00%)

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[kentr0w]

lgtm