Fix for wrong plurals with multiple domains

When translations from different catalogs are being merged, there is a
workaround to copy the `plurals()` function from the catalog to the combined
`translations` object. If the locale catalog doesn't exist for a
particular domain, a default value is copied, overriding the proper
`plurals()`function. This results in the plural version of a string not
being translated for languages with different plural values.

See for instance `test_gettext.py::test_plurals_different_domains`: the
Japanese locale exists only in the `messages` domain (`translations`
folder) but not on the `myapp` one (`translations_different_domain`
folder).

First reported in ckan/ckan#8258

flask_babel/__init__.py

@@ -630,7 +630,7 @@ def get_translations(self):
                 # does not copy _info, plural(), or any other instance variables
                 # populated by GNUTranslations. We probably want to stop using
                 # `support.Translations.merge` entirely.
-                if hasattr(catalog, 'plural'):
+                if catalog.info() and hasattr(catalog, 'plural'):
                     translations.plural = catalog.plural
 
             cache[str(locale), self.domain[0]] = translations