docs: fix collection metadata removal description (#9907)

Setting metadata to an empty object does not clear it: the update merges
key by key, so an empty object is a no-op (and over gRPC an empty map is
indistinguishable from an absent one). Per-key removal via null values is
the mechanism that was actually implemented and tested in #7123.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/redoc/master/openapi.json

@@ -11365,7 +11365,7 @@
             ]
           },
           "metadata": {
-            "description": "Metadata to update for the collection. If provided, this will merge with existing metadata. To remove metadata, set it to an empty object.",
+            "description": "Metadata to update for the collection. If provided, this will merge with existing metadata. Individual keys can be removed by setting their value to `null`.",
             "anyOf": [
               {
                 "$ref": "#/components/schemas/Payload"

lib/storage/src/content_manager/collection_meta_ops.rs

@@ -315,7 +315,7 @@ pub struct UpdateCollection {
     #[validate(nested)]
     pub strict_mode_config: Option<StrictModeConfig>,
     /// Metadata to update for the collection. If provided, this will merge with existing metadata.
-    /// To remove metadata, set it to an empty object.
+    /// Individual keys can be removed by setting their value to `null`.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub metadata: Option<Payload>,
 }