Empty keys are not validated on upload

[Zverikk]

Vitalts shared a way to upload an object with empty key:

1. In any text editor write " \tvalue" (space, TAB, "value") or "\t\tvalue" (two TABs and a value), copy it to the clipboard
2. Select an object in JOSM and press Ctrl+Shift+V (paste tags).

You'll get " =value" key-value pair, which can be uploaded.
Example object: ​https://www.openstreetmap.org/way/316197424