fix missing root node edge case and use nil snapshot to work with 1.9.15

environments/example.toml

@@ -0,0 +1,6 @@
+[database]
+    name     = "vulcanize_public" # $DATABASE_NAME
+    hostname = "localhost" # $DATABASE_HOSTNAME
+    port     = 5432 # $DATABASE_PORT
+    user     = "postgres" # $DATABASE_USER
+    password = "" # $DATABASE_PASSWORD

environments/testing.toml

@@ -0,0 +1,4 @@
+[database]
+  name     = "vulcanize_testing"
+  hostname = "localhost"
+  port     = 5432