Fix credential library mocks (#2161)

* fix: 🐛 vault cred library mocks

addons/api/mirage/config.js

@@ -632,7 +632,23 @@ function routes() {
       { queryParams: { credential_store_id: credentialStoreId } },
     ) => credentialLibraries.where({ credentialStoreId }),
   );
-  this.get('/credential-libraries/:id');
+  this.get(
+    '/credential-libraries/:id',
+    function ({ credentialLibraries }, { params: { id } }) {
+      const library = credentialLibraries.find(id);
+      // BE API only updates these fields if there's an update mask, otherwise they remain unmodified
+      // but we don't need this flow in our mocks, so we are deleting fields with null value
+      const { credentialMappingOverrides } = library;
+      if (Object.keys(credentialMappingOverrides || {}).length) {
+        Object.keys(credentialMappingOverrides).forEach((key) => {
+          if (credentialMappingOverrides[key] === null) {
+            delete credentialMappingOverrides[key];
+          }
+        });
+      }
+      return library;
+    },
+  );
   this.post(
     '/credential-libraries',
     function ({ credentialStores, credentialLibraries }) {