Allow ActiveRecord's #insert to work with an empty hash of attributes when using MySQL

Here's what happens: when calling `Book.insert({})`, the following
exception is raised:
```
/Users/sto/.gem/ruby/ruby-3.3.5/gems/irb-1.14.1/lib/irb.rb:1260:in
`full_message': Trilogy::ProtocolError: 1054: Unknown column '' in
'field list' (ActiveRecord::StatementInvalid)
```

This is due to Rails generating the following SQL query:
```sql
INSERT INTO `posts` (`created_at`,`updated_at`) VALUES (CURRENT_TIMESTAMP(6), CURRENT_TIMESTAMP(6)) AS `posts_values` ON DUPLICATE KEY UPDATE ``=`posts_values`.`` /*application='MysqlDefaultTime'*/
```

This simple commit should allow it to work, though I must admit that I
haven't tested all scenarios locally, hoping that the test suite might
catch any problem in CI.

Nonetheless, I think it's reasonable to skip appending `ON DUPLICATE KEY
UPDATE` using `no_op_column` when `no_op_column` is made of a
non-existent column name.

Author: davidstosik

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Fix incorrect SQL query when passing an empty hash to `ActiveRecord::Base.insert`.
+
+    *David Stosik*
+
 *   Allow to save records with polymorphic join tables that have inverse of specified.
 
     *Markus Doits*

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -628,7 +628,7 @@ def default_index_type?(index) # :nodoc:
       end
 
       def build_insert_sql(insert) # :nodoc:
-        no_op_column = quote_column_name(insert.keys.first)
+        no_op_column = quote_column_name(insert.keys.first) if insert.keys.first
 
         # MySQL 8.0.19 replaces `VALUES(<expression>)` clauses with row and column alias names, see https://dev.mysql.com/worklog/task/?id=6312 .
         # then MySQL 8.0.20 deprecates the `VALUES(<expression>)` see https://dev.mysql.com/worklog/task/?id=13325 .
@@ -637,7 +637,9 @@ def build_insert_sql(insert) # :nodoc:
           sql = +"INSERT #{insert.into} #{insert.values_list} AS #{values_alias}"
 
           if insert.skip_duplicates?
-            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{values_alias}.#{no_op_column}"
+            if no_op_column
+              sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{values_alias}.#{no_op_column}"
+            end
           elsif insert.update_duplicates?
             if insert.raw_update_sql?
               sql = +"INSERT #{insert.into} #{insert.values_list} ON DUPLICATE KEY UPDATE #{insert.raw_update_sql}"
@@ -651,7 +653,9 @@ def build_insert_sql(insert) # :nodoc:
           sql = +"INSERT #{insert.into} #{insert.values_list}"
 
           if insert.skip_duplicates?
-            sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{no_op_column}"
+            if no_op_column
+              sql << " ON DUPLICATE KEY UPDATE #{no_op_column}=#{no_op_column}"
+            end
           elsif insert.update_duplicates?
             sql << " ON DUPLICATE KEY UPDATE "
             if insert.raw_update_sql?

activerecord/test/cases/insert_all_test.rb

@@ -149,6 +149,14 @@ def test_insert_all_generates_correct_sql
         Book.insert_all [{ id: 1, name: "Agile Web Development with Rails" }]
       end
     end
+
+    def test_insert_all_succeeds_when_passed_no_attributes
+      skip unless supports_insert_on_duplicate_skip?
+
+      assert_nothing_raised do
+        Book.insert_all [{}]
+      end
+    end
   end
 
   def test_insert_all_with_skip_duplicates_and_autonumber_id_not_given