python: test also named AbstractImporter APIs with no file opened.

Code coverage says red!

src/python/magnum/test/test_trade.py

@@ -605,6 +605,8 @@ class Importer(unittest.TestCase):
 
         with self.assertRaisesRegex(AssertionError, "no file opened"):
             importer.mesh(0)
+        with self.assertRaisesRegex(AssertionError, "no file opened"):
+            importer.mesh('')
 
         with self.assertRaisesRegex(AssertionError, "no file opened"):
             importer.image1d_count
@@ -636,10 +638,16 @@ class Importer(unittest.TestCase):
 
         with self.assertRaisesRegex(AssertionError, "no file opened"):
             importer.image1d(0)
+        with self.assertRaisesRegex(AssertionError, "no file opened"):
+            importer.image1d('')
         with self.assertRaisesRegex(AssertionError, "no file opened"):
             importer.image2d(0)
+        with self.assertRaisesRegex(AssertionError, "no file opened"):
+            importer.image2d('')
         with self.assertRaisesRegex(AssertionError, "no file opened"):
             importer.image3d(0)
+        with self.assertRaisesRegex(AssertionError, "no file opened"):
+            importer.image3d('')
 
     def test_index_oob(self):
         importer = trade.ImporterManager().load_and_instantiate('StbImageImporter')