is_string check on array key when serializeNulls is needed. 

Hi, I had a situation where I needed to serialize an Array of Arrays.

For example
[
["a","b","c"],
[4,null,5],
[1,2,3]
]

Even with serialize nulls configured, it would still strip the null out of the 2nd array.

This was due to the is_string check on the array key (in this case, the number 2) causing the if statement to be true despite the requirement to serialise nulls.

I could not think of a reason for the is_string check on the array key, so I removed it, resolving my issue.

Please advise the intended purpose of the is_string check ? And how I might modify the code to incorporate the scenario of numeric array keys ?
If you agree with what I've done, please accept.