Don't fail if baseline doesn't exist in PlainDetekt

[BraisGabin]

Fixes #3355

At work I realised that we had some modules without it's baseline and they were working as expected. I just figured out why: we have this code

detekt/detekt-gradle-plugin/src/main/kotlin/io/gitlab/arturbosch/detekt/internal/DetektJvm.kt

Lines 30 to 34 in a260383

	// If a baseline file is configured as input file, it must exist to be configured, otherwise the task fails.
	// We try to find the configured baseline or alternatively a specific variant matching this task.
	extension.baseline?.existingVariantOrBaseFile(sourceSet.name)?.let { baselineFile ->
	baseline.set(layout.file(project.provider { baselineFile }))
	}

for jvm and android but we don't for plain detekt. In this PR I just add that code in detekt too.

[codecov]

Codecov Report

Merging #3429 (ca0ceed) into master (a260383) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #3429      +/-   ##
============================================
- Coverage     80.37%   80.35%   -0.03%     
  Complexity     2763     2763              
============================================
  Files           450      450              
  Lines          8335     8336       +1     
  Branches       1599     1599              
============================================
- Hits           6699     6698       -1     
- Misses          774      775       +1     
- Partials        862      863       +1     

Impacted Files	Coverage Δ	Complexity Δ
...o/gitlab/arturbosch/detekt/internal/DetektPlain.kt	76.47% <0.00%> (-11.03%)	5.00 <0.00> (ø)
.../gitlab/arturbosch/detekt/internal/FileMangling.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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