tests: Exercise gdbus-codegen --interface-info-header with empty input

Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/tests/codegen.py

@@ -367,6 +367,31 @@ class TestCodegen(unittest.TestCase):
 G_BEGIN_DECLS
 
 
+G_END_DECLS
+
+#endif /* __STDOUT__ */""".format(
+                **result.subs
+            ),
+            result.out.strip(),
+        )
+
+    def test_empty_interface_info_header(self):
+        """Test generating a header with an empty interface file."""
+        result = self.runCodegenWithInterface(
+            "", "--output", "-", "--interface-info-header"
+        )
+        self.assertEqual("", result.err)
+        self.assertEqual(
+            """{standard_top_comment}
+
+#ifndef __STDOUT__
+#define __STDOUT__
+
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+
 G_END_DECLS
 
 #endif /* __STDOUT__ */""".format(