Support predicate pruning on `Expr::Case` expressions

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
In certain situations , IOx is likely going to make predicates that look like the following

```sql
CASE
WHEN col IS NULL THEN ''
ELSE col
END
```
that basically map `null` to the empty string

We would like to use such predicates in order to prune out Chunks (or parquet record groups)

**Describe the solution you'd like**
I would like `CaseExpr` to be added to the list of expression types supported by predicate pruning in `build_predicate_expression`:
https://github.com/apache/arrow-datafusion/blob/03075d5f4b3fdfd8f82144fcd409418832a4bf69/datafusion/src/physical_optimizer/pruning.rs#L640-L699

The tricky bit of this PR would be figuring out what the transformation is and in what circumstances it can be applied

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
See https://github.com/influxdata/influxdb_iox/pull/3557 for more details

Start in datafusion/src/physical_optimizer/pruning.rs at build_predicate_expression, especially the expression types currently supported around the linked lines. Determine the safe transformation and applicable conditions for Expr::Case, then verify that predicates like the NULL-to-empty-string example can prune chunks or parquet record groups without incorrect results.

Written by the indexing model from the issue text.