Fall back when a translation is stored as an empty string

[shreyanshj10]

When a translation is stored as an empty string in i18n, which happens easily through the admin with ActiveLanguageMixin, the fallback works when reading the attribute but not in queries.

TranslatedVirtualField.__get__() already skips an empty translation and continues down the fallback chain (test_fallback_getting_CharField covers that). as_expression() builds a COALESCE, which only skips NULL, so values(), filter() and order_by() hand back the empty string instead of the fallback value.

I wrapped every i18n lookup in the coalesce chain in NULLIF(<lookup>, ''), so an empty translation is treated as missing there too. The lookup for the default language points at the original field and is the last resort of the chain, so I left it as it is; that keeps the current result when every value is empty.

Added FallbackEmptyStringTest in tests/test_querysets.py, covering values_list(), filter() and a model using fallback_language_field.

Fixes #139