use alternative for all falsey values in format strings

… and not just None (#525)

It would be better to consistently use None for all non-existent
fields and/or fields without a valid value, but this is a good
enough workaround for now.

gallery_dl/util.py

@@ -646,7 +646,7 @@ class Formatter():
                     obj = kwdict[key]
                     for func in funcs:
                         obj = func(obj)
-                    if obj is not None:
+                    if obj:
                         break
                 except Exception:
                     pass