IBM Db2 provider: skip None-valued extra parameters to avoid KEY=None in connection strings

[ShubhamKapoor992]

Description

Skip None-valued extra parameters when building Db2 connection strings and URIs

When extra connection parameters contain keys with None values (e.g. from a partially-filled connection form), the previous code would emit them as the literal string "None" in the Db2 connection string (KEY=None;) and in the SQLAlchemy URI query string (?KEY=None). Both cases result in either a connection error or silently passing a bad value to the driver.

This fix skips None-valued keys in both get_conn() (connection string builder) and get_uri() (SQLAlchemy URI builder), so only explicitly configured parameters reach the Db2 driver.

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Note: The change is small and self-contained and the provider isn't released nor used so I have not created the Git Issue.

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[ColtenOuO]

Thanks for the fix -- skipping None-valued extras in get_conn()/get_uri() looks correct and avoids emitting KEY=None to the Db2 driver.

I think there have a few things before this is ready to merge:

1. The PR title should clearly describe the specific changes rather than just mentioning which provider were modified.
2. I think we need to add a test to prevent the same issue from happening again in the future.
3. To keep the branch clean, we should rebase onto main instead of merging.
4. Similar to the PR title, commit messages should explain the reasoning and the solution behind the changes in detail, so that future contributors touching similar files can quickly understand the context.