schema: Support References without type

There are some examples in the OSV database which have untyped
references. Currently these fail to parse. For example:
https://www.googleapis.com/download/storage/v1/b/osv-vulnerabilities/o/Debian%2FCVE-2018-11212.json?generation=1758701673567991&alt=media

Author: Jarrah-TLR

src/schema.rs

@@ -395,7 +395,7 @@ pub struct Affected {
 /// The type of reference information that has been provided. Examples include
 /// links to the original report, external advisories, or information about the
 /// fix.
-#[derive(Clone, Debug, Serialize, Deserialize)]
+#[derive(Clone, Debug, Default, Serialize, Deserialize)]
 #[serde(rename_all = "UPPERCASE")]
 #[non_exhaustive]
 pub enum ReferenceType {
@@ -430,6 +430,7 @@ pub enum ReferenceType {
     /// A report, typically on a bug or issue tracker, of the vulnerability.
     Report,
 
+    #[default]
     #[serde(rename = "NONE")]
     Undefined,
 
@@ -441,7 +442,7 @@ pub enum ReferenceType {
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct Reference {
     /// The type of reference this URL points to.
-    #[serde(rename = "type")]
+    #[serde(rename = "type", default)]
     pub reference_type: ReferenceType,
 
     /// The url where more information can be obtained about