Fix EtsCfgToDot

[Lipen]

This PR fixes EtsCfg.toDot method: (1) escape " in labels, (2) showing value-less returns simply as "return" instead of "return null".

[github-actions]

Lifecycle test results

78 tests  ±0   78 ✅ ±0   1m 26s ⏱️ -1s
 7 suites ±0    0 💤 ±0 
 7 files   ±0    0 ❌ ±0 

Results for commit feed79d. ± Comparison against base commit 6db1ff5.

[github-actions]

Test results on JDK 19

 72 files  ±0   72 suites  ±0   4m 42s ⏱️ +3s
628 tests ±0  618 ✅ ±0  10 💤 ±0  0 ❌ ±0 
698 runs  ±0  688 ✅ ±0  10 💤 ±0  0 ❌ ±0 

Results for commit feed79d. ± Comparison against base commit 6db1ff5.

[github-actions]

Test results on JDK 8

 72 files  ±0   72 suites  ±0   4m 53s ⏱️ +7s
628 tests ±0  615 ✅ ±0  13 💤 ±0  0 ❌ ±0 
698 runs  ±0  685 ✅ ±0  13 💤 ±0  0 ❌ ±0 

Results for commit feed79d. ± Comparison against base commit 6db1ff5.

[github-actions]

Test results on JDK 11

 72 files  ±0   72 suites  ±0   5m 17s ⏱️ +5s
628 tests ±0  620 ✅ ±0  8 💤 ±0  0 ❌ ±0 
698 runs  ±0  690 ✅ ±0  8 💤 ±0  0 ❌ ±0 

Results for commit feed79d. ± Comparison against base commit 6db1ff5.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.19%. Comparing base (6db1ff5) to head (feed79d).
Report is 3 commits behind head on neo.

Files with missing lines	Patch %	Lines
...rc/main/kotlin/org/jacodb/ets/utils/EtsCfgToDot.kt	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                neo     #307      +/-   ##
============================================
- Coverage     69.19%   69.19%   -0.01%     
  Complexity     2474     2474              
============================================
  Files           269      269              
  Lines         15441    15442       +1     
  Branches       2281     2281              
============================================
  Hits          10685    10685              
  Misses         3829     3829              
- Partials        927      928       +1     

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