Fixed increment bug that changes expiration to forever

A bug was introduced recently when changing:

```
if ($existing = $this->get($key)) {
```

to

```
if (! isset($this->storage[$key])) {
```

When attempting to increment a value that starts from zero (0), before it worked correctly in that it was simply returning true in that a value was set. However, now when `$this->get($key)` returns 0 since we are incrementing from 0, it evaluates as false and thus changes expiration to "forever". This causes issues for testing rate limiting because nothing ever "expires".

src/Illuminate/Cache/ArrayStore.php

@@ -93,7 +93,7 @@ public function put($key, $value, $seconds)
      */
     public function increment($key, $value = 1)
     {
-        if ($existing = $this->get($key)) {
+        if (! is_null($existing = $this->get($key))) {
             return tap(((int) $existing) + $value, function ($incremented) use ($key) {
                 $value = $this->serializesValues ? serialize($incremented) : $incremented;