Instead of asking "why should searching for an empty string do x?" (does the average user of this site even know what "empty string" means) or "why do x when a site whose core purpose, structure, and design philosophy are different does not?", try asking "why does clicking that little magnifying glass button not take me to the dedicated search page?"
One person's "baffling design decision" is another person's "neat little shortcut"
Someone has noted in a previous thread that implementation is as simple as removing the "required" attribute from the search box. The thread was so long ago, though, I don't know if it got a staff response or what it was.