[build-scripts] Do not write plugins.xml if the plugins list is empty

This fixes an exception
```
'non_bundled_plugins' build step failed:
java.nio.file.NoSuchFileException:
/mnt/agent/work/89e70edcf4afcfd6/out/idea-ue/artifacts/IU-plugins/auto-uploading/plugins.xml
```
thrown from `generatePluginRepositoryMetaFile`

GitOrigin-RevId: ec76222f5d024dafb9b41b6040be71fd43055681

platform/build-scripts/src/org/jetbrains/intellij/build/impl/DistributionJARsBuilder.kt

@@ -471,8 +471,14 @@ internal suspend fun buildNonBundledPlugins(
 
     if (prepareCustomPluginRepository) {
       val list = pluginSpecs.sortedBy { it.pluginZip }
-      generatePluginRepositoryMetaFile(list, context.nonBundledPlugins, context)
-      generatePluginRepositoryMetaFile(list.filter { it.pluginZip.startsWith(context.nonBundledPluginsToBePublished) }, context.nonBundledPluginsToBePublished, context)
+      if (list.isNotEmpty()) {
+        generatePluginRepositoryMetaFile(list, context.nonBundledPlugins, context)
+      }
+
+      val pluginsToBePublished = list.filter { it.pluginZip.startsWith(context.nonBundledPluginsToBePublished) }
+      if (pluginsToBePublished.isNotEmpty()) {
+        generatePluginRepositoryMetaFile(pluginsToBePublished, context.nonBundledPluginsToBePublished, context)
+      }
     }
 
     validatePlugins(context, pluginSpecs)

platform/build-scripts/src/org/jetbrains/intellij/build/impl/PluginRepositoryXmlGenerator.kt

@@ -14,11 +14,13 @@ import java.util.*
 import kotlin.io.path.createDirectories
 
 internal fun generatePluginRepositoryMetaFile(pluginSpecs: List<PluginRepositorySpec>, targetDir: Path, context: BuildContext): Path {
+  require(pluginSpecs.isNotEmpty())
   val categories = TreeMap<String, MutableList<Plugin>>()
   for (spec in pluginSpecs) {
     val p = readPlugin(spec.pluginZip, spec.pluginXml, context.buildNumber, targetDir)
     categories.computeIfAbsent(p.category) { mutableListOf() }.add(p)
   }
+  Files.createDirectories(targetDir)
   val result = targetDir.resolve("plugins.xml")
   writePluginsXml(result, categories)
   return result