Include empty import file in npm installation (#1485)

Fixes missing line from #1347

Author: ifiokjr

packages/firebase/package.json

@@ -19,6 +19,7 @@
     "/firebase*.js",
     "/firebase*.map",
     "/index.d.ts",
+    "/empty-import.d.ts",
     "/externs"
   ],
   "repository": {