Update patterns.md with empty property pattern details (#49385)

* Update patterns.md with empty property pattern details

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

* Add code example for empty property pattern

Added a code example demonstrating the use of an empty property pattern with variable creation in C#.

* Move sample to snippets.

Per conversation on #49385

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

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Bill Wagner <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

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

@@ -182,6 +182,10 @@ 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 specifically 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`.
+
+:::code language="csharp" source="snippets/patterns/PropertyPattern.cs" id="EmptyPropertyPattern":::
+
 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":::

docs/csharp/language-reference/operators/snippets/patterns/PropertyPattern.cs

@@ -5,8 +5,27 @@ public static class PropertyPattern
     public static void Examples()
     {
         WithTypeCheck();
+
+        // <EmptyPropertyPattern>
+        if (GetSomeNullableStringValue() is { } nonNullValue) // Empty property pattern with variable creation
+        {
+            Console.WriteLine("NotNull:" + nonNullValue);
+        }
+        else
+        {
+            nonNullValue = "NullFallback"; // we can access the variable here.
+            Console.WriteLine("it was null, here's the fallback: " + nonNullValue);
+        }
+        // </EmptyPropertyPattern>
+
     }
 
+    private static string? GetSomeNullableStringValue()
+    {
+        // Simulate getting a nullable string value.
+        return DateTime.Now.Ticks % 2 == 0 ? "Hello, World!" : null;
+    }
+    
     // <BasicExample>
     static bool IsConferenceDay(DateTime date) => date is { Year: 2020, Month: 5, Day: 19 or 20 or 21 };
     // </BasicExample>