fix: Db2 fixes for padded char identification and default IfNull replacement token

[nj1973]

Description of changes

We had a customer with issues on Db2 with a CHAR padded primary key.

To fix this I needed to resolve part of issue 1470, although only for Db2 so this PR does NOT close that issue. Unfortunately the fix only covers table level validations. There does not appear to be a way to get the raw data type for a custom query via the IBM Db2 driver.

The small changes to files below are to remove deprecation warnings when customers run DVT, they should not change functionality:

• operations.py: FutureWarning: Series.view is deprecated and will be removed in a future version. Use ``astype`` as an alternative to change the dtype.
• combiner.py: FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an inplace method.

Issues to be closed

Closes #1638

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines

[nj1973]

I've put this back to draft while I move some dependent changes from this PR to new issue #1661

[nj1973]

/gcbrun