Update patterns.md with empty property pattern details

Clarify the behavior of the empty property pattern in C#.

Author: AdmiralSnyder

docs/csharp/language-reference/operators/patterns.md

@@ -182,6 +182,8 @@ You can also add a run-time type check and a variable declaration to a property
 
 :::code language="csharp" source="snippets/patterns/PropertyPattern.cs" id="WithTypeCheck":::
 
+This specifially means that the *empty* property pattern `is { }` matches everything non-null, and can be used instead of the `is not null` to create a variable: `somethingPossiblyNull is { } somethingDefinitelyNotNull`.
+
 A property pattern is a recursive pattern. You can use any pattern as a nested pattern. Use a property pattern to match parts of data against nested patterns, as the following example shows:
 
 :::code language="csharp" source="snippets/patterns/PropertyPattern.cs" id="RecursivePropertyPattern":::