Add @SkipWhenEmpty and @IgnoreEmptyDirectories for better incremental…

[Frisch12]

… builds.

Improve incremental build performance by adding skip annotations to tasks that should skip execution when inputs are empty:

• PlantumlTask: Override getSource() with @SkipWhenEmpty to skip when no PlantUML source files exist
• GenerateCodeTask: Add @SkipWhenEmpty and @IgnoreEmptyDirectories to inputDir
• DescriptorGeneratorTask: Add @SkipWhenEmpty to source and classes directories
• MkDocsBuild/MkDocsServe: Add @IgnoreEmptyDirectories to themeDir, add @PathSensitive for MkDocsServe consistency

These annotations allow Gradle to skip task execution when inputs are empty, improving build performance and enabling better build cache usage.

[Frisch12]

Automated maintenance: Resolved merge conflicts with main branch.

Conflicts were in:

• GenerateCodeTask.java — kept both @SkipWhenEmpty/@IgnoreEmptyDirectories (from this PR) and @PathSensitive(PathSensitivity.RELATIVE) (from main)
• DescriptorGeneratorTask.java — kept both @SkipWhenEmpty (from this PR) and @PathSensitive/@Classpath (from main)

Both sets of annotations serve complementary purposes and are non-conflicting. CI should re-run automatically. This PR is ready for manual review.

[Frisch12]

Automated maintenance summary:

1. Merge conflicts resolved: MkDocsServe.java — kept both @IgnoreEmptyDirectories (from this PR) and @PathSensitive(PathSensitivity.RELATIVE) (from main). Both annotations serve complementary purposes.

2. Code fixes applied:

   • DescriptorGeneratorTask: Removed @SkipWhenEmpty from getClassesDirectories(). Having @SkipWhenEmpty on both getSourceDirectories() and getClassesDirectories() would cause incorrect task skipping when only one input is empty. Kept it only on getSourceDirectories() as the primary driver.
   • PlantumlTask: Added @IgnoreEmptyDirectories to getSource() for consistent cache key behavior, matching how other @CacheableTask classes handle @SkipWhenEmpty inputs.

3. CI status: Pipelines triggered and running. Waiting for results.