[formatter] use value of last alternative (#4492)

fixes {fieldname|''} evaluating to the value of 'keywords-default'
instead of an empty string

gallery_dl/formatter.py

@@ -182,9 +182,10 @@ class StringFormatter():
                     if obj:
                         break
                 except Exception:
-                    pass
+                    obj = None
             else:
-                obj = self.default
+                if obj is None:
+                    obj = self.default
             return fmt(obj)
         return wrap
 

test/test_formatter.py

@@ -340,6 +340,8 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{'foobar'[:3]}", value)
         self._run_test("{z|'foo'}"     , value)
         self._run_test("{z|''|'foo'}"  , value)
+        self._run_test("{z|''}"        , "")
+        self._run_test("{''|''}"       , "")
 
         self._run_test("{_lit[foo]}"       , value)
         self._run_test("{_lit[foo]!u}"     , value.upper())