Keep NVARCHAR(MAX) length as -1 (doctrine#6251)

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|      Q       |   A
|------------- | -----------
| Type         | bug
| Fixed issues | doctrine#6038

#### Summary

As described in the mentioned issue, `-0.5` from `-1 / 2` will raise
type error when trying to `setLength`

```
Argument #1 ($length) must be of type ?int, float given
```

https://github.com/doctrine/dbal/blob/8bc0d9ae42f20af54120396ef7a1b3248479c2b5/src/Schema/Column.php#L78

This PR check if length is `-1` and keep it as is before perform
division.

src/Schema/SQLServerSchemaManager.php

@@ -133,12 +133,20 @@ protected function _getPortableTableColumnDefinition($tableColumn)
 
         switch ($dbType) {
             case 'nchar':
-            case 'nvarchar':
             case 'ntext':
                 // Unicode data requires 2 bytes per character
                 $length /= 2;
                 break;
 
+            case 'nvarchar':
+                if ($length === -1) {
+                    break;
+                }
+
+                // Unicode data requires 2 bytes per character
+                $length /= 2;
+                break;
+
             case 'varchar':
                 // TEXT type is returned as VARCHAR(MAX) with a length of -1
                 if ($length === -1) {

tests/Functional/Schema/SQLServerSchemaManagerTest.php

@@ -262,4 +262,19 @@ public function testPkOrdering(): void
         self::assertEquals('colB', $columns[0]);
         self::assertEquals('colA', $columns[1]);
     }
+
+    public function testNvarcharMaxIsLengthMinus1(): void
+    {
+        $sql = 'CREATE TABLE test_nvarchar_max (
+            col_nvarchar_max NVARCHAR(MAX),
+            col_nvarchar NVARCHAR(128)
+        )';
+
+        $this->connection->executeStatement($sql);
+
+        $table = $this->schemaManager->introspectTable('test_nvarchar_max');
+
+        self::assertSame(-1, $table->getColumn('col_nvarchar_max')->getLength());
+        self::assertSame(128, $table->getColumn('col_nvarchar')->getLength());
+    }
 }