org-agenda queries for absent properties

[Jens Schmidt <jschmidt4gnu@vodafonemail.de>]

The following was initially meant as bug report until I understood that
it's actually some sort of feature ...

Consider the following todo list:

-------------------- todos.org --------------------
* TODO relax

* TODO do this
:PROPERTIES:
:patchday: 202302
:END:

* TODO do that
:PROPERTIES:
:patchday: 202305
:END:

* TODO work harder
:PROPERTIES:
:patchday: 202308
:END:
-------------------- todos.org --------------------

When I use an agenda query

   patchday>=202305

on that I get the result:

   Headlines with TAGS match: patchday>=202305
   Press ‘C-u r’ to search again
     todos:      TODO do that
     todos:      TODO work harder

However for the following:

   patchday<=202305

I get

   Headlines with TAGS match: patchday<=202305
   Press ‘C-u r’ to search again
     todos:      TODO relax
     todos:      TODO do this
     todos:      TODO do that

since the absent property "patchday" on the "relax" todo entry is
defaulted to value zero, obviously.

So I would like to have something easier to type (and remember! why not
"!=" or "/="?) then

   patchday<>0&patchday<=202305

How about starred agenda property operators that match only if the
operand property is actually present, which would result in this query:

   patchday<=*202305

Section <info:org#Matching tags and properties> does not seem to provide
any information on "definedness" expressions for properties.

Thanks.