Add skip_serializing_if to PullRequest optional fields

PullRequest now omits insecure and stream from serialized JSON when
unset, consistent with all other request types in the codebase.
Previously these fields serialized as null.

src/types/pull.rs

@@ -3,7 +3,9 @@ use serde::{Deserialize, Serialize};
 #[derive(Debug, Serialize, Deserialize)]
 pub struct PullRequest {
     pub model: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub insecure: Option<bool>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub stream: Option<bool>,
 }