Fix variables JSON string

I don't have any variables set, and when running a mutation; I get the following error:

```
Error SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at VueComponent._callee$ (graphql.vue?f047:505)
    at tryCatch (runtime.js?96cf:45)
    at Generator.invoke [as _invoke] (runtime.js?96cf:271)
    at Generator.prototype.<computed> [as next] (runtime.js?96cf:97)
    at asyncGeneratorStep (asyncToGenerator.js?1da1:3)
    at _next (asyncToGenerator.js?1da1:25)
    at eval (asyncToGenerator.js?1da1:32)
    at new Promise (<anonymous>)
    at VueComponent.eval (asyncToGenerator.js?1da1:21)
```

Setting the `variablesJSONString` variable to the correct default (`{}`) fixes this issue :)

store/state.js

@@ -20,7 +20,7 @@ export default () => ({
   gql: {
     url: "https://rickandmortyapi.com/graphql",
     headers: [],
-    variablesJSONString: "",
+    variablesJSONString: "{}",
     query: ""
   },
   oauth2: {