fix(redis): treat explicit db=None as database index 0

[ahmed5145]

Description

RedisInstrumentor reads the database index from the connection's
connection_kwargs via conn_kwargs.get("db", 0). dict.get only falls
back to the default when the key is missing — if db is present and set to
None (which redis-py allows, e.g. Redis(..., db=None) or some from_url
paths), the value stays None. That None was then passed straight to the
database-index attribute, producing:

Invalid type NoneType for attribute 'db.redis.database_index' value. Expected one of ['bool', 'str', 'bytes', 'int', 'float'] or a sequence of those types

This coerces an explicit db=None to 0 (Redis' default database), in both
_extract_conn_attributes and the pipeline span-name fallback in
_build_span_name, so no invalid attribute is emitted.

Fixes #1905

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Added test_attributes_db_none, which sets db=None in the client's
  connection kwargs and asserts the span reports db.redis.database_index == 0
  with no invalid-attribute warning.
• Ran the redis instrumentation test suite locally (test_attributes_db_none
  and test_attributes_default pass).

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated