[Gradle, JS] Add nullable custom fields

^KT-42395 fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/Kotlin2JsGradlePluginIT.kt

@@ -6,6 +6,8 @@
 package org.jetbrains.kotlin.gradle
 
 import com.google.gson.Gson
+import com.google.gson.JsonNull
+import com.google.gson.JsonObject
 import org.gradle.api.logging.LogLevel
 import org.gradle.api.logging.configuration.WarningMode
 import org.gradle.testkit.runner.BuildResult
@@ -1112,6 +1114,30 @@ abstract class AbstractKotlin2JsGradlePluginIT(protected val irBackend: Boolean)
         }
     }
 
+    @DisplayName("package.json custom fields")
+    @GradleTest
+    fun testPackageJsonCustomField(gradleVersion: GradleVersion) {
+        project("kotlin-js-browser-project", gradleVersion) {
+            buildGradleKts.modify(::transformBuildScriptWithPluginsDsl)
+
+            build("rootPackageJson") {
+                assertTasksExecuted(":app:packageJson")
+
+                val jso = projectPath.resolve("build/js/packages/kotlin-js-browser-app")
+                    .resolve(NpmProject.PACKAGE_JSON)
+                    .let {
+                        Gson().fromJson(it.readText(), JsonObject::class.java)
+                    }
+
+                assertEquals(1, jso.get("customField1").asJsonObject.get("one").asInt)
+                assertEquals(2, jso.get("customField1").asJsonObject.get("two").asInt)
+                assertEquals(JsonNull.INSTANCE, jso.get("customField2").asJsonNull)
+                assertEquals(JsonNull.INSTANCE, jso.get("customField3").asJsonNull)
+                assertEquals(JsonNull.INSTANCE, jso.get("customField4").asJsonObject.get("foo").asJsonNull)
+            }
+        }
+    }
+
     @DisplayName("kotlin/js compiler warning")
     @GradleTest
     fun testKotlinJsCompilerWarn(gradleVersion: GradleVersion) {

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlin-js-browser-project/app/build.gradle.kts

@@ -47,7 +47,10 @@ kotlin {
 
         compilations.named("main") {
             packageJson {
-                customField("customField", mapOf("one" to 1, "two" to 2))
+                customField("customField1", mapOf("one" to 1, "two" to 2))
+                customField("customField2", null)
+                customField("customField3" to null)
+                customField("customField4", mapOf("foo" to null))
             }
         }
     }

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/js/npm/PackageJson.kt

@@ -58,11 +58,11 @@ class PackageJson(
     val bundledDependencies = mutableListOf<String>()
         get() = field ?: mutableListOf()
 
-    fun customField(pair: Pair<String, Any>) {
+    fun customField(pair: Pair<String, Any?>) {
         customFields[pair.first] = pair.second
     }
 
-    fun customField(key: String, value: Any) {
+    fun customField(key: String, value: Any?) {
         customFields[key] = value
     }
 
@@ -92,6 +92,7 @@ class PackageJson(
         val gson = GsonBuilder()
             .setPrettyPrinting()
             .disableHtmlEscaping()
+            .serializeNulls()
             .addSerializationExclusionStrategy(
                 object : ExclusionStrategy {
                     override fun shouldSkipField(f: FieldAttributes?): Boolean =