Avoid NPE on kotlin 1.7.20

With kotlin 1.7.20, some of the source sets (that are nullable properties)
are actually null. In order to avoid crash, we create an empty source set
and use it as a fallback.

library/gradle-plugin/src/main/java/io/mehow/laboratory/gradle/SourceSetContribution.kt

@@ -40,9 +40,7 @@ private fun TaskProvider<out Task>.contributeToAndroid(dir: File, project: Proje
   }
   val sources = extension.sourceSets.associate { set -> set.name to set.kotlin }
   for (variant in extension.variants) {
-    val kotlinSourceSet = requireNotNull(sources[variant.name]) {
-      "Did not find Kotlin source set for variant ${variant.name}"
-    }
+    val kotlinSourceSet = sources[variant.name] ?: project.createEmptySourceSet(variant.name)
     kotlinSourceSet.srcDir(dir.toRelativeString(project.projectDir))
     variant.addJavaSourceFoldersToModel(dir)
 
@@ -58,9 +56,7 @@ private fun TaskProvider<out Task>.contributeToAndroid(dir: File, project: Proje
 
 private fun contributeToKotlin(dir: File, project: Project) {
   val sourceSets = project.property("sourceSets") as SourceSetContainer
-  val kotlinSourceSet = requireNotNull(sourceSets.getByName("main").kotlin) {
-    "Did not find Kotlin source set"
-  }
+  val kotlinSourceSet = sourceSets.getByName("main").kotlin ?: project.createEmptySourceSet("empty")
   kotlinSourceSet.srcDir(dir)
 }
 
@@ -83,3 +79,6 @@ private val Any.kotlin: SourceDirectorySet?
   }
 
 private fun Any.getConvention(name: String) = (this as HasConvention).convention.plugins[name]
+
+private fun Project.createEmptySourceSet(name: String) =
+  objects.sourceDirectorySet(name, "Empty kotlin source set")