Improve the error message for empty mainClassName

[Goooler]

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• CHANGELOG's "Unreleased" section has been updated, if applicable.

[Copilot]

Pull Request Overview

This PR improves error handling for the Shadow plugin by providing a clearer error message when the main class is not specified or is empty. The changes ensure that users receive helpful feedback instead of encountering unclear failures when they forget to set the application's main class.

• Enhanced error message clarity for missing or empty main class configuration
• Added comprehensive test coverage for the empty main class scenario
• Updated changelog to document the improvement

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ShadowApplicationPlugin.kt	Replaced the implicit property input with explicit validation and improved error message for empty main class
ApplicationPluginTest.kt	Added test case to verify the new error message when main class is not set
docs/changes/README.md	Updated changelog to document the error message improvement

Comments suppressed due to low confidence (1)

src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/ShadowApplicationPlugin.kt:131

• The removal of the inputs property declaration may affect Gradle's up-to-date checking and incremental builds. Consider adding the property back or ensuring that task inputs are properly declared elsewhere to maintain build performance.

    tasks.shadowJar.configure { task ->

[Goooler]

This will result in the shadowJar task not being re-run after applicationExtension.mainClass changed.

[Goooler]

#1603