docs: add skip_null_values in configuration and upgrade-guide (#1618)

Author: mdieudonne

core/configuration.md

@@ -322,7 +322,9 @@ api_platform:
                 xkey:
                     glue: ' '
 
-        normalization_context: ~
+        normalization_context:
+            # Default value to omit null values in conformance with the JSON Merge Patch RFC.
+            skip_null_values: true
         denormalization_context: ~
         swagger_context: ~
         openapi_context: ~

core/upgrade-guide.md

@@ -15,6 +15,14 @@ api_platform:
 This will be the default value in 3.0, in 2.7 it's left to `true` so that nothing breaks by updating.
 By doing so you won't get access to legacy services and this will probably break things on code using `api-platform/core:2.6`.
 
+In 3.0, in conformance with the JSON Merge Patch RFC, the default value of the `skip_null_values` property is `true` which means that from now on `null` values are omitted during serialization.
+```yaml
+api_platform:
+  defaults:
+    normalization_context:
+      skip_null_values: true
+```
+
 ### I'm Migrating From 2.6 and Want to Prepare For 3.0
 
 1. Update the code to 2.7: `composer require api-platform/core:^2.7`