fix default sync function dry run

Author: RIT3shSapata

db/crud.go

@@ -1711,12 +1711,15 @@ func (db *DatabaseCollectionWithUser) SyncFnDryrun(ctx context.Context, newDoc,
 	}
 	var output *channels.ChannelMapperOutput
 	var syncErr error
-	if syncFn == "" {
+	if syncFn == "" && db.ChannelMapper != nil {
 		output, err = db.ChannelMapper.MapToChannelsAndAccess(ctx, mutableBody, string(oldDoc._rawBody), metaMap, syncOptions)
 		if err != nil {
 			return nil, &base.SyncFnDryRunError{Err: err}
 		}
 	} else {
+		if syncFn == "" {
+			syncFn = channels.DocChannelsSyncFunction
+		}
 		jsTimeout := time.Duration(base.DefaultJavascriptTimeoutSecs) * time.Second
 		syncRunner, err := channels.NewSyncRunner(ctx, syncFn, jsTimeout)
 		if err != nil {

rest/diagnostic_doc_api_test.go

@@ -1144,6 +1144,30 @@ func TestSyncFuncDryRun(t *testing.T) {
 			expectedOutput:  SyncFnDryRun{},
 			expectedStatus:  http.StatusBadRequest,
 		},
+		{
+			name:        "no_custom_sync_func-default_db_sync_func-doc_body-no_existing_doc-no_doc_id",
+			document:    map[string]any{"channels": "chanNew"},
+			existingDoc: false,
+			expectedOutput: SyncFnDryRun{
+				Channels: base.SetFromArray([]string{"chanNew"}),
+				Access:   channels.AccessMap{},
+				Roles:    channels.AccessMap{},
+			},
+			expectedStatus: http.StatusOK,
+		},
+		{
+			name:            "no_custom_sync_func-default_db_sync_func-no_doc_body-existing_doc-doc_id",
+			docID:           "doc22",
+			existingDoc:     true,
+			existingDocID:   "doc22",
+			existingDocBody: `{"channels": "chanNew"}`,
+			expectedOutput: SyncFnDryRun{
+				Channels: base.SetFromArray([]string{"chanNew"}),
+				Access:   channels.AccessMap{},
+				Roles:    channels.AccessMap{},
+			},
+			expectedStatus: http.StatusOK,
+		},
 	}
 
 	for _, test := range tests {