Fix `NoMethodError: undefined method `fields' for nil:NilClass`

Currently `exec_query` raises `NoMethodError` when executing no result
queries (`INSERT`, `UPDATE`, `DELETE`, and all DDL) in mysql2 adapter.

```
irb(main):002:0> conn.execute("create table t(a int)")
   (43.3ms)  create table t(a int)
=> nil
irb(main):003:0> conn.execute("insert into t values (1)")
   (19.3ms)  insert into t values (1)
=> nil
irb(main):004:0> conn.exec_query("insert into t values (1)")
  SQL (28.6ms)  insert into t values (1)
NoMethodError: undefined method `fields' for nil:NilClass
```

1 files changed, 1 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index 57d8867bb4..ed8cfae657 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -131,7 +131,7 @@ module ActiveRecord
       def exec_query(sql, name = 'SQL', binds = [], prepare: false)
         result = execute(sql, name)
         @connection.next_result while @connection.more_results?
-        ActiveRecord::Result.new(result.fields, result.to_a)
+        ActiveRecord::Result.new(result.fields, result.to_a) if result
       end
 
       def exec_insert(sql, name, binds, pk = nil, sequence_name = nil)