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GH-34165: [Python] Extension array data type should default to the storage type if to_pandas_dtype is not implemented (#34559)

### Rationale for this change
Method `to_pandas` fails with `KeyError` if a table has an extension array as a column with extension dtype not having `to_pandas_dtype` defined. In this cases we should fall back to storage type of the extension array.

### What changes are included in this PR?
Changes in `arrow_to_pandas.cc` at:
- `GetBlockType` for `ConvertTableToPandas`
- `ConvertChunkedArrayToPandas`
* Closes: #34165

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>