[5.6] Fix: If the expiration is null, set it permanently when increments in file cache driver

[eddiclin]

I meet a bug when using increment of file cache driver.

Reproduce

Use file cache driver, config it in the .env

CACHE_DRIVER=file

Cache::increment('key');    // return 1
Cache::increment('key');    // ERROR: return 1

Fix

If the expiration is null, set it permanently when increments.

After fix:

Cache::increment('key');    // return 1
Cache::increment('key');    // return 2