Sparse array of strings

Hi,
how do you recommend to store a sparse array of strings? E.g. I have an array: [str1, null, null, null, str2, null, null, null, null, str3, null, null] Is this ok, does it take similar space as a dense array [str1, str2, str3]? Or should I better to convert it to a dense array and store indexes somehow?
Thanks