Document how conditions treat empty and null values

[joshdaugherty]

The conditions page doesn't say what a condition does when its value is empty or null. Two different things happen, depending on how the value is passed. Both are easy to hit when the value comes from something that can be blank, such as a query-string value, an optional field or an empty picker.

Measured on statamic/cms v6.26.0, with the collection tag on a collection of 75 published entries:

Template	Value the tag receives	Result
type:is="", or type:is="{{ get:type }}" when type is absent from the query string	''	The condition is skipped: all 75 entries are returned
:type:is="some_var" when some_var is null or not set	null	Filters to entries where type is null, so only entries without that field are returned
:type:is="some_var" when some_var is ''	''	The condition is skipped, as in the first row

The skip comes from Statamic\Tags\Concerns\QueriesConditions::queryableConditionParams(), which rejects any parameter whose value is ''. A null is not rejected, so it reaches $query->where($field, null).

Both behaviors seem reasonable. What's missing is a note saying so, because the two forms look interchangeable and do opposite things when the value is blank. The first lists everything; the second can list nothing, for example on blueprint, which every entry has.

Suggested wording, under Using a variable reference:

If a condition's value is an empty string, the condition is skipped and the results are not filtered by it. If a variable reference (:field:is="variable") resolves to null, the condition filters for entries where that field is empty.

Docs PR with that wording: #2001.