Cache: better fix for #5860 introduced in #5863

Cache: better fix for #5860 introduced in #5863
The setter `setObjectKey` allows only `string` and `NULL` but thinking more of it you are right as of before an empty string or `"0"` was handled the same as `NULL`. So I think `"0"` should be a valid key but an empty string should not be allowed by setter.
-> I'll update the PR and add a test case for it