[O] BUG: org-set-property should not accept an empty string as property

[Julien Cubizolles]

According to
http://orgmode.org/worg/dev/org-syntax.html#Node_Properties, the NAME of
a property cannot be an empty string. However, the interactive version
of org-set-property accepts such a string, resulting in a malformed
drawer. When giving an empty string at the prompt for the property NAME,
org-set-property should instead gracefully exit without creating a new
property in the drawer.