Recommendation 4.3.1 avoid nullable types

[kentr0w]

Actions checklist

• Implemented Rule, added Warnings
• Added tests on checks
• Added tests on fixers
• Updated diktat-analysis.yml
• Updated available-rules.md
• closes Recommendation 4.3.1: avoid declaration of variables of nullable types, especially of types from Kotlin stdlib. #294

Fixme

Add fix logic

[codecov]

Codecov Report

Merging #296 into master will decrease coverage by 0.06%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##             master     #296      +/-   ##
============================================
- Coverage     82.86%   82.79%   -0.07%     
- Complexity     1144     1182      +38     
============================================
  Files            57       58       +1     
  Lines          2865     2935      +70     
  Branches        917      930      +13     
============================================
+ Hits           2374     2430      +56     
- Misses          162      169       +7     
- Partials        329      336       +7     

Flag	Coverage Δ	Complexity Δ
#unittests	82.79% <80.00%> (-0.07%)	1182.00 <38.00> (+38.00)	⬇️

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)
.../org/cqfn/diktat/ruleset/rules/NullableTypeRule.kt	79.41% <79.41%> (ø)	38.00 <38.00> (?)
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	96.93% <100.00%> (+0.03%)	9.00 <0.00> (ø)
...cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt	97.91% <100.00%> (+0.04%)	3.00 <0.00> (ø)

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[petertrr]

Take a look at snapshot check:
[INFO] [ktlint] /home/runner/work/diKTat/diKTat/diktat-common/src/test/kotlin/org/cqfn/diktat/test/ConfigReaderTest.kt:10:30: [NULLABLE_PROPERTY_TYPE] try to avoid use of nullable types: don't use nullable type (cannot be auto-corrected)
You should check assigned value before issuing a warning. You absolutely shouldn't suggest to change type if right hand side expression returns nullable value.

[petertrr]

Please change initial value for Char, apart from this LGTM

[petertrr]

Wait, I found this:
[INFO] [ktlint] /home/runner/work/diKTat/diKTat/diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/IdentifierNaming.kt:258:29: [NULLABLE_PROPERTY_TYPE] try to avoid use of nullable types: don't use nullable type (cannot be auto-corrected)
and this:
[INFO] [ktlint] /home/runner/work/diKTat/diKTat/diktat-rules/src/test/kotlin/org/cqfn/diktat/util/DiktatRuleSetProvider4Test.kt:20:34: [NULLABLE_PROPERTY_TYPE] try to avoid use of nullable types: don't use nullable type (cannot be auto-corrected)

Both warnings are clearly false-positive