Fix return type for CollectionRef::new

gir doesn't seem to generate this correctly. I have no clue why, there
are certainly some functions where nullable=1 causes an Option return.

rust-bindings/rust/gir-files/OSTree-1.0.gir

@@ -917,7 +917,7 @@ indicating a ref name which is not globally unique.</doc>
 @collection_id is %NULL, this is equivalent to a plain ref name string (not a
 refspec; no remote name is included), which can be used for non-P2P
 operations.</doc>
-        <return-value transfer-ownership="full">
+        <return-value transfer-ownership="full" nullable="1">
           <doc xml:space="preserve">a new #OstreeCollectionRef</doc>
           <type name="CollectionRef" c:type="OstreeCollectionRef*"/>
         </return-value>

rust-bindings/rust/src/collection_ref/mod.rs

@@ -20,7 +20,8 @@ glib_wrapper! {
 
 impl CollectionRef {
     #[cfg(any(feature = "v2018_6", feature = "dox"))]
-    pub fn new<'a, P: Into<Option<&'a str>>>(collection_id: P, ref_name: &str) -> CollectionRef {
+    // CHANGE: return type CollectionRef to Option<CollectionRef>
+    pub fn new<'a, P: Into<Option<&'a str>>>(collection_id: P, ref_name: &str) -> Option<CollectionRef> {
         let collection_id = collection_id.into();
         let collection_id = collection_id.to_glib_none();
         unsafe {
@@ -60,3 +61,6 @@ impl hash::Hash for CollectionRef {
         hash::Hash::hash(&self.hash(), state)
     }
 }
+
+#[cfg(test)]
+mod tests;