In English, in the middle of a sentence if data is being left blank (such as an unknown date, or a name to be filled in at a later time), commonly you see ???, ___ (underscore(s)), or -- to mark this. You can see examples in books such as Mary Shelly's Frankenstein, where the year is always indicated as 18--, hiding the last two digits. In Pokemon Gold, Silver, and Crystal (although not the most formal example of language), your rival was called ??? until you named him.
In Japanese and Korean, 〇〇 (or simply 〇 at times, though this is easy to confuse with 0 in Japanese) is used to mark "something" in that place.
In Chinese, what is/are the most common ways to suggest that data is missing or incomplete?