refactor: Metadata now uses custom deserializer

serde_yaml 0.9.7 resulted in a regression,
accepting `metadata:` (empty metadata mappings)
instead of returning Err.

See dtolnay/serde-yaml#304 and dtolnay/serde-yaml#312

This change uses a custom deserializer to error
in this case.

src/specification.rs

@@ -2336,10 +2336,27 @@ impl DemeDefaults {
 /// ```
 #[derive(Clone, Default, Debug, Serialize, Deserialize, Eq, PartialEq)]
 pub struct Metadata {
-    #[serde(flatten)]
+    #[serde(flatten, deserialize_with = "require_non_empty_metadata")]
     metadata: std::collections::BTreeMap<String, serde_yaml::Value>,
 }
 
+fn require_non_empty_metadata<'de, D>(
+    deserializer: D,
+) -> Result<std::collections::BTreeMap<String, serde_yaml::Value>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    let buf = std::collections::BTreeMap::<String, serde_yaml::Value>::deserialize(deserializer)?;
+
+    if !buf.is_empty() {
+        Ok(buf)
+    } else {
+        Err(serde::de::Error::custom(
+            "metadata: cannot be an empty mapping".to_string(),
+        ))
+    }
+}
+
 impl Metadata {
     /// `true` if metadata is present, `false` otherwise
     fn is_empty(&self) -> bool {