Document how the Stache treats a missing field in where queries

[joshdaugherty]

The Content Queries page does not say what whereNull, whereNotNull and where do when an entry has no value at all for the field being queried. On the flat-file driver the answer is not obvious, and it matters most for toggle fields, where a missing key usually means "never set".

This adds a short note to the WhereNull section, scoped to the Stache.

Measured on statamic/cms v6.26.0 against entries holding a missing key, null, false, true, '' and a non-empty string:

Query	Matches
whereNull	missing key, null
whereNotNull	false, true, '', non-empty string
where(field, false) and where(field, '')	missing key, null, false, ''
where(field, true)	true only
where(field, '!=', false)	missing key, null, true, non-empty string

The cause is in the Stache query builder: every entry's value is resolved before filtering, so a missing key becomes null. filterWhereNull tests === null, and filterTestEquals compares strtolower($item ?? '') with strtolower($value ?? ''), which folds null, false and '' together.