[#4629] Allow caching null values from callbacks

- Implemented test case for #4629
- Used fix suggested in original report
Merge branch 'hotfix/4624'

Close #4624
Merge branch 'hotfix/4629' of git://github.com/weierophinney/zf2 into hotfix/4629
Merge branch 'hotfix/4629' into develop

Forward port #4629