Fix nullable javaPackagePrefix used as @Input, resulting in build failure

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -236,9 +236,11 @@ open class KotlinCompile : AbstractKotlinCompile<K2JVMCompilerArguments>(), Kotl
      *
      * Example: a Java source file with `package com.example.my.package` is located in directory `src/main/java/my/package`.
      * Then, for the Kotlin compilation to locate the source file, use package prefix `"com.example"` */
-    @Input
     var javaPackagePrefix: String? = null
 
+    @get:Input
+    internal val javaPackagePrefixInputString get() = javaPackagePrefix ?: ""
+
     internal val pluginOptions = CompilerPluginOptions()
     internal var artifactDifferenceRegistryProvider: ArtifactDifferenceRegistryProvider? = null
     internal var artifactFile: File? = null