CMake fix for projects without plugins (#59343)

This install step fails if the list is empty, which is the case for projects without plugins (or Dart-only plugins, so flutter/plugins presubmits are currently broken).

packages/flutter_tools/templates/app/linux.tmpl/CMakeLists.txt.tmpl

@@ -68,8 +68,11 @@ install(FILES "${FLUTTER_ICU_DATA_FILE}" DESTINATION "${INSTALL_BUNDLE_DATA_DIR}
 install(FILES "${FLUTTER_LIBRARY}" DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
   COMPONENT Runtime)
 
-install(FILES "${PLUGIN_BUNDLED_LIBRARIES}" DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
+if(PLUGIN_BUNDLED_LIBRARIES)
+  install(FILES "${PLUGIN_BUNDLED_LIBRARIES}"
+    DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
     COMPONENT Runtime)
+endif()
 
 # Fully re-copy the assets directory on each build to avoid having stale files
 # from a previous install.
@@ -81,7 +84,7 @@ install(DIRECTORY "${PROJECT_BUILD_DIR}/${FLUTTER_ASSET_DIR_NAME}"
   DESTINATION "${INSTALL_BUNDLE_DATA_DIR}" COMPONENT Runtime)
 
 # Install the AOT library on non-Debug builds only.
-if (NOT CMAKE_BUILD_TYPE MATCHES "Debug")
+if(NOT CMAKE_BUILD_TYPE MATCHES "Debug")
   install(FILES "${AOT_LIBRARY}" DESTINATION "${INSTALL_BUNDLE_LIB_DIR}"
     COMPONENT Runtime)
 endif()