add missing exit codes to ensure Symfony 5 compatibility

lib/Doctrine/DBAL/Tools/Console/Command/ImportCommand.php

@@ -58,7 +58,7 @@ EOT
         $fileNames = $input->getArgument('file');
 
         if ($fileNames === null) {
-            return null;
+            return 0;
         }
 
         foreach ((array) $fileNames as $fileName) {
@@ -133,6 +133,6 @@ EOT
             }
         }
 
-        return null;
+        return 0;
     }
 }

lib/Doctrine/DBAL/Tools/Console/Command/RunSqlCommand.php

@@ -68,5 +68,7 @@ EOT
         }
 
         $output->write(Dumper::dump($resultSet, (int) $depth));
+
+        return 0;
     }
 }

tests/Doctrine/Tests/DBAL/Tools/Console/RunSqlCommandTest.php

@@ -70,10 +70,11 @@ class RunSqlCommandTest extends TestCase
     {
         $this->expectConnectionFetchAll();
 
-        $this->commandTester->execute([
+        $exitCode = $this->commandTester->execute([
             'command' => $this->command->getName(),
             'sql' => 'SELECT 1',
         ]);
+        $this->assertSame(0, $exitCode);
 
         self::assertRegExp('@int.*1.*@', $this->commandTester->getDisplay());
         self::assertRegExp('@array.*1.*@', $this->commandTester->getDisplay());