IcingaCommandArgument: render set_if without format

Icinga DSL support was introduced in 348cb1aa6 and works fine, but breaks
rendering of legacy arguments with set_if_format = NULL. This should default
to 'string', as we should IMO not force NOT NULL in the DB in this case.

refs #12153
fixes #12886

library/Director/Objects/IcingaCommandArgument.php

@@ -163,12 +163,13 @@ class IcingaCommandArgument extends IcingaObject
 
         if ($this->set_if) {
             switch ($this->set_if_format) {
-                case 'string':
-                    $data['set_if'] = c::renderString($this->set_if);
-                    break;
                 case 'expression':
                     $data['set_if'] = c::renderExpression($this->set_if);
                     break;
+                case 'string':
+                default:
+                    $data['set_if'] = c::renderString($this->set_if);
+                    break;
             }
         }
 

test/php/library/Director/Objects/IcingaCommandTest.php

@@ -174,6 +174,22 @@ class IcingaCommandTest extends BaseTestCase
         );
     }
 
+    public function testSimpleSetIfIsRendered()
+    {
+        $command = $this->command();
+        $command->command = 'bla';
+        $command->arguments = array(
+            '-a' => (object) array(
+                'set_if' => '$a$',
+            )
+        );
+
+        $this->assertEquals(
+            $this->loadRendered('command7'),
+            (string) $command
+        );
+    }
+
     protected function command()
     {
         return IcingaCommand::create(

test/php/library/Director/Objects/rendered/command7.out

@@ -0,0 +1,9 @@
+object CheckCommand "___TEST___command" {
+    command = [ PluginDir + "/bla" ]
+    arguments = {
+        "-a" = {
+            set_if = "$a$"
+        }
+    }
+}
+