add check for empty outdated output (npm 6.10 issue)

changelog.txt

@@ -10,3 +10,4 @@
 * Fix emulation of https functions.
 * Fix bug where calling `clearFirestoreData` against the Firestore Emulator fails if more than 500 documents exist.
 * Fixes a bug where `admin.firestore()` and `app.firestore()` behaved differently.
+* Fix issue with npm `6.10` where an internal version check of the `firebase-functions` SDK caused a crash.

src/checkFirebaseSDKVersion.js

@@ -1,5 +1,6 @@
 "use strict";
 
+var _ = require("lodash");
 var clc = require("cli-color");
 var path = require("path");
 
@@ -37,7 +38,7 @@ module.exports = function(options) {
       resolve();
     }
   }).then(function(output) {
-    if (!output) {
+    if (!output || _.isEmpty(output)) {
       return;
     }
     var wanted = output["firebase-functions"].wanted;