Pie Chart crashes when Name Column is empty

### Feature Summary

The Pie Chart descriptor validates its Value Column but does not validate its required Name Column. An empty Name Column passes workflow code generation and becomes an empty pandas column name at execution time.

Before: an empty Name Column reaches pandas and fails with a KeyError.

After: the descriptor rejects the configuration with `Name Column cannot be empty` before Python execution.

Reproduction evidence:

1. Create a Pie Chart operator.
2. Set Value Column to a numeric column such as `amount`.
3. Leave Name Column empty.
4. Generate or run the operator code.
5. Observe that the generated `dropna` call looks up an empty column name and fails.

Version and commit evidence:

1.3.0-incubating-SNAPSHOT on main

**Commit Hash (Optional)**
70c21145887920528d7d5540e3fb790b43e8b759

**Relevant log output**

pandas raises a KeyError for the empty column name.

### Proposed Solution or Design

After: the descriptor rejects the configuration with `Name Column cannot be empty` before Python execution.

### Affected Area

Workflow Engine (Amber)