KT-37587 Karma. Revert input check for 'webpack.config.d' directory

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/webpack/KotlinWebpack.kt

@@ -108,7 +108,7 @@ open class KotlinWebpack : DefaultTask(), RequiresNpmDependencies {
         get() = destinationDirectory.resolve(outputFileName)
 
     open val configDirectory: File?
-        @Optional @InputDirectory get() = project.projectDir.resolve("webpack.config.d")
+        @Optional @InputDirectory get() = project.projectDir.resolve("webpack.config.d").takeIf { it.isDirectory }
 
     @Input
     var report: Boolean = false