Fix condition on Ascii String for SQL Server (#6389)

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|      Q       |   A
|------------- | -----------
| Type         | bug
| Fixed issues | #6245

#### Summary

This MR fixes a condition on SQL Server where `fixed` is always defined
as `true` or `false` so `return sprintf('VARCHAR(%d)', $length ?? 255);`
is always ignored.

Author: Yovach

src/Platforms/SQLServerPlatform.php

@@ -1306,7 +1306,7 @@ public function getAsciiStringTypeDeclarationSQL(array $column): string
     {
         $length = $column['length'] ?? null;
 
-        if (! isset($column['fixed'])) {
+        if (empty($column['fixed'])) {
             return sprintf('VARCHAR(%d)', $length ?? 255);
         }
 

tests/Platforms/SQLServerPlatformTest.php

@@ -123,17 +123,55 @@ public function testGeneratesTypeDeclarationsForStrings(): void
             'NVARCHAR(50)',
             $this->platform->getStringTypeDeclarationSQL(['length' => 50]),
         );
+        self::assertEquals(
+            'NVARCHAR(50)',
+            $this->platform->getStringTypeDeclarationSQL(
+                ['length' => 50, 'fixed' => false],
+            ),
+        );
         self::assertEquals(
             'NVARCHAR(255)',
             $this->platform->getStringTypeDeclarationSQL([]),
         );
+        self::assertEquals(
+            'NVARCHAR(255)',
+            $this->platform->getStringTypeDeclarationSQL(['fixed' => false]),
+        );
         self::assertSame('VARCHAR(MAX)', $this->platform->getClobTypeDeclarationSQL([]));
         self::assertSame(
             'VARCHAR(MAX)',
             $this->platform->getClobTypeDeclarationSQL(['length' => 5, 'fixed' => true]),
         );
     }
 
+    public function testGeneratesTypeDeclarationsForAsciiStrings(): void
+    {
+        self::assertEquals(
+            'CHAR(10)',
+            $this->platform->getAsciiStringTypeDeclarationSQL(
+                ['length' => 10, 'fixed' => true],
+            ),
+        );
+        self::assertEquals(
+            'VARCHAR(50)',
+            $this->platform->getAsciiStringTypeDeclarationSQL(['length' => 50]),
+        );
+        self::assertEquals(
+            'VARCHAR(50)',
+            $this->platform->getAsciiStringTypeDeclarationSQL(
+                ['length' => 50, 'fixed' => false],
+            ),
+        );
+        self::assertEquals(
+            'VARCHAR(255)',
+            $this->platform->getAsciiStringTypeDeclarationSQL([]),
+        );
+        self::assertEquals(
+            'VARCHAR(255)',
+            $this->platform->getAsciiStringTypeDeclarationSQL(['fixed' => false]),
+        );
+    }
+
     public function testPrefersIdentityColumns(): void
     {
         self::assertTrue($this->platform->prefersIdentityColumns());