fix: add default inMemoryCacheOptions

vue-cli-plugin-apollo now defaults it to null, which won't trigger
default parameter fallback, so this triggers another bug in
apollo-cache-inmemory 1.6.0 https://github.com/apollographql/apollo-client/blob/787be614419c39db1dc79ac132a18730c8d7af1b/packages/apollo-cache-inmemory/src/inMemoryCache.ts#L136
causing "Cannot read property 'freezeResults' of null" error.

(cherry picked from commit ac8b2b3)

packages/@vue/cli-ui/src/vue-apollo.js

@@ -24,6 +24,7 @@ if (typeof endpoint === 'undefined') {
 
 // Config
 const options = {
+  inMemoryCacheOptions: {},
   wsEndpoint: endpoint,
   persisting: false,
   websocketsOnly: true,