fix: handle null turbo_controller without spurious warnings

[jasonstjohn]

## Fix explanation

When `turbo_controller: null` is explicitly set in a tuning template's generator section, Badger was producing a misleading warning:

Could not find compatible class for  in field turbo_controller


The warning appeared repeatedly (once per generator) because the code checked `defaults.get(field)` which returns `None` both when:
1. The field is missing from the config (legitimate case to warn about)
2. The field is explicitly set to `null` (expected behavior, no warning needed)

## Root cause

In `badger/gui/components/pydantic_editor.py`, the code used `defaults.get(field, {})` which:
- Returns `{}` when the key doesn't exist → triggers warning for missing field
- Returns `None` when key exists but value is `null` → `None` was then treated as empty dict, resulting in empty string for `name`, causing repeated warnings

## The fix

1. Changed `defaults.get(field, {})` to `defaults.get(field)` to properly detect missing fields
2. When `special_item_dict` is empty (from `null`), default the name to `"null"` instead of `""`

This way:
- Missing field → warning displayed (correct)
- `turbo_controller: null` → no warning, "null" selected (correct)

### Diff

```diff
--- a/badger/gui/components/pydantic_editor.py
+++ b/badger/gui/components/pydantic_editor.py
@@ -754,15 +754,23 @@ class PydanticEditor(QWidget):
             field: str = "turbo_controller"
             # Get value without default - if key doesn't exist, get() returns None
             # If key exists but value is None (from null in YAML), it's also None
             # We need to distinguish these cases for the warning
             special_item_dict: dict[str, Any] | None = defaults.get(field)
 
             if special_item_dict is None:
-                logger.warning(
+                # Check if key exists in dict - if not, warn; if yes, it's explicitly null
+                if field not in defaults:
+                    logger.warning(
                         f"Generator has {field} set but no compatible {field} exists in defaults. "
                         "Item has likely been filtered out from not being included in defaults."
                     )
+                special_item_dict = {}
-
+ 
             special_item_dict["vocs"] = self.vocs.model_dump()
 
-            name = special_item_dict.get("name", "")
+            # Use the name from dict, or "null" if dict is empty (null was set)
+            name = special_item_dict.get("name", "null" if not special_item_dict else "")