fix(mysql/ddl): DEFAULT NULL must imply nullable column

When a field carries `default:null` (e.g. SoftDeletedAt) GORM sets
DefaultValue="null" but the column.Nullable flag was never set to true
unless the Go type was a pointer or the NULLABLE tag was present.

This produced the contradictory DDL:
  `deleted_at` timestamp DEFAULT null NOT NULL
which MySQL/MariaDB rejects with Error 1067.

Fix: after the pointer/NULLABLE check, force column.Nullable=true
whenever column.Default is "null" (case-insensitive). A column that
defaults to NULL cannot be NOT NULL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: getevo

lib/mysql/ddl.go

@@ -142,6 +142,13 @@ func fromStatementToTable(stmt *gorm.Statement) ddlTable {
 			}
 		}
 
+		// A column whose default value is NULL must be nullable — regardless of
+		// whether the Go type is a pointer. Emitting DEFAULT NULL NOT NULL is
+		// contradictory and causes MySQL/MariaDB error 1067.
+		if strings.EqualFold(column.Default, "null") {
+			column.Nullable = true
+		}
+
 		if (column.Type == "TIMESTAMP" || column.Type == "timestamp") && column.Default == "" && !column.Nullable {
 			column.Default = "CURRENT_TIMESTAMP"
 		}