Remove SDK Defaults for Namespace Parameter (#546)

## Summary
Removes SDK-imposed default values for the `namespace` parameter in GRPC
methods, ensuring the SDK doesn't override API defaults. This change
allows the API to handle namespace defaults appropriately as it moves
toward `"__default__"` as the default namespace value.

## Changes

### GRPC Method Updates
- Updated `pinecone/grpc/resources/vector_grpc.py`: Changed
`upsert_from_dataframe` method signature from `namespace: str = ""` to
`namespace: Optional[str] = None`
- Updated `pinecone/grpc/index_grpc.py`: Changed `upsert_from_dataframe`
method signature from `namespace: str = ""` to `namespace: Optional[str]
= None`

### Behavior Verification
- All REST API methods already correctly use `Optional[str] = None` for
namespace parameters
- When `namespace=None`: Parameter is omitted from request bodies (via
`parse_non_empty_args`), allowing the API to apply its default
- When `namespace=""`: Parameter is included as empty string in request
(explicit user choice, passed through unmodified)
- When `namespace="some_namespace"`: Parameter is included in request as
expected

## Impact
- **GRPC `upsert_from_dataframe` methods**: Now default to `None`
instead of `""`, allowing the API to handle namespace defaults
- **Backward compatibility**: No breaking changes - methods still accept
all the same values, but default behavior now defers to the API
- **API flexibility**: The API can now apply its own default namespace
handling (e.g., `"__default__"`) without SDK interference

## Rationale
The API is moving toward `"__default__"` as the default namespace value.
By removing SDK-imposed defaults (empty string), we ensure:
- The SDK doesn't override API defaults
- When users don't specify a namespace, the API can apply its own
default handling
- Explicit empty string values from users are still passed through as
intended
- The SDK remains neutral regarding namespace defaults, allowing the API
to evolve its default behavior

## Testing
- All db data integration tests pass (104 passed, 18 skipped)
- Verified that `namespace=None` omits the parameter from requests
- Verified that `namespace=""` passes through as empty string
- Verified that explicit namespace values work correctly

## Files Changed
- `pinecone/grpc/resources/vector_grpc.py`
- `pinecone/grpc/index_grpc.py`

Author: jhamon

pinecone/grpc/index_grpc.py

@@ -236,7 +236,7 @@ def _upsert_batch(
     def upsert_from_dataframe(
         self,
         df: Any,
-        namespace: str = "",
+        namespace: str | None = None,
         batch_size: int = 500,
         use_async_requests: bool = True,
         show_progress: bool = True,

pinecone/grpc/resources/vector_grpc.py

@@ -174,7 +174,7 @@ def _upsert_batch(
     def upsert_from_dataframe(
         self,
         df,
-        namespace: str = "",
+        namespace: Optional[str] = None,
         batch_size: int = 500,
         use_async_requests: bool = True,
         show_progress: bool = True,