Preserve explicit empty query string values

[sylvesterkaczmarek]

Summary

Preserve explicitly empty string query parameters instead of silently dropping them from the request URL.

Querystring._stringify_item() currently converts a primitive value to a string and then drops it whenever the serialized result is falsy. That intentionally omits None, but it also conflates "" with absence:

• {"key": None} -> parameter omitted
• {"key": ""} -> parameter also omitted

HTTP APIs can distinguish a missing parameter from an explicitly blank one (key=), so callers currently cannot express the latter through the SDK query serializer.

The same issue affects nested values and repeat/bracket array entries containing an empty string.

Fix

Make omission depend on the original value being None, not on the truthiness of its serialized representation.

Existing behavior is preserved for None. Explicit empty strings now serialize as key=. All non-empty strings, numbers, booleans, nested mappings, and existing array formatting continue through the same serialization paths as before.

Regression coverage

Adds focused tests covering:

• a top-level empty string -> blank=;
• a nested empty string -> filter[name]=;
• repeat-array empty strings remaining present while None entries stay omitted;
• bracket-array empty strings with the same behavior;
• the existing top-level None omission contract.

The production change is confined to the query-string serializer.