Fix Cache::many() with small numeric keys

[AlexKarpan]

Cache::many([1,2,3]) for an empty cache returns [1 => 2, 2 => 3, 3 => null] instead of [1 => null, 2 => null, 3 => null].
String keys like this \Cache::many(['1','2','3']) make no difference.

The issue is that Illuminate\Cache\Repository tries to set default values for missing keys, and in case of small numeric keys it treats those as positions: e.g. when $keys = [1, 2, 3] the value of $keys[1] is 2.

This code in this PR tests if $keys is not an associative array (in other words, it doesn't contain the defaults, just the keys) and if so, just returns null for missed cache keys.