MSSQL migration alterColumn on a column with default value NULL creat…

…es wrong CONSTRAINT (#488)

* MSSQL migration alterColumn on a column with default value NULL creates wrong CONSTRAINT
yiisoft/yii2#18617

* styleci

src/Schema/AbstractColumnSchemaBuilder.php

@@ -224,26 +224,37 @@ protected function buildUniqueString(): string
     }
 
     /**
-     * Builds the default value specification for the column.
+     * Return the default value for the column.
      *
-     * @return string A string containing the DEFAULT keyword and the default value.
+     * @return string|null string with default value of column.
      */
-    protected function buildDefaultString(): string
+    protected function buildDefaultValue()
     {
         if ($this->default === null) {
-            return $this->isNotNull === false ? ' DEFAULT NULL' : '';
+            return $this->isNotNull === false ? 'NULL' : null;
         }
 
-        $string = ' DEFAULT ';
-
-        $string .= match (gettype($this->default)) {
+        return match (gettype($this->default)) {
             'object', 'integer' => (string)$this->default,
             'double' => StringHelper::normalizeFloat((string)$this->default),
             'boolean' => $this->default ? 'TRUE' : 'FALSE',
             default => "'$this->default'",
         };
+    }
+
+    /**
+     * Builds the default value specification for the column.
+     *
+     * @return string A string containing the DEFAULT keyword and the default value.
+     */
+    protected function buildDefaultString(): string
+    {
+        $defaultValue = $this->buildDefaultValue();
+        if ($defaultValue === null) {
+            return '';
+        }
 
-        return $string;
+        return ' DEFAULT ' . $defaultValue;
     }
 
     /**