archive/plugins/meson.build: do not generate empty library

If no archive library was found, return from the "plugins" directory
without creating "libarchive_plugins.a".  Empty static libraries are
unsupported on some operating systems such as macOS.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1650

src/archive/plugins/meson.build

@@ -22,6 +22,10 @@ if libzzip_dep.found()
   found_archive_plugin = true
 endif
 
+if not found_archive_plugin
+  subdir_done()
+endif
+
 archive_plugins = static_library(
   'archive_plugins',
   archive_plugins_sources,