fix: set default properly when modifying

lib/migration_generator/migration_generator.ex

@@ -471,6 +471,17 @@ defmodule AshPostgres.MigrationGenerator do
     else
       string
     end
+  rescue
+    exception ->
+      IO.puts("""
+      Exception while formatting:
+
+      #{inspect(exception)}
+
+      #{inspect(string)}
+      """)
+
+      reraise exception, __STACKTRACE__
   end
 
   defp streamline(ops, acc \\ [])
@@ -619,6 +630,9 @@ defmodule AshPostgres.MigrationGenerator do
     name in keys || (multitenancy.attribute && name == multitenancy.attribute)
   end
 
+  defp after?(%Operation.AddUniqueIndex{table: table}, %Operation.RemoveUniqueIndex{table: table}),
+    do: true
+
   defp after?(
          %Operation.AddUniqueIndex{identity: %{keys: keys}, table: table},
          %Operation.AlterAttribute{table: table, new_attribute: %{name: name}}

lib/migration_generator/operation.ex

@@ -124,7 +124,11 @@ defmodule AshPostgres.MigrationGenerator.Operation do
 
       default =
         if attribute.default != old_attribute.default do
-          ", default: #{attribute.default}"
+          if is_nil(attribute.default) do
+            ", default: nil"
+          else
+            ", default: #{attribute.default}"
+          end
         end
 
       null =