fix: type hinting in collection.py (#688)

* fix: Fix type hinting in collection.py

Mostly adding `| None` on all fields that may have `None` as a default value.

* fix: Use Union in type hinting

Use Union to be compatible with 3.7

---------

Co-authored-by: Cedric Khin <[email protected]>
Co-authored-by: Mariatta Wijaya <[email protected]>

google/cloud/firestore_v1/collection.py

@@ -25,7 +25,7 @@
 from google.cloud.firestore_v1 import aggregation
 from google.cloud.firestore_v1.watch import Watch
 from google.cloud.firestore_v1 import document
-from typing import Any, Callable, Generator, Tuple
+from typing import Any, Callable, Generator, Tuple, Union
 
 # Types needed only for Type Hints
 from google.cloud.firestore_v1.transaction import Transaction
@@ -79,9 +79,9 @@ def _aggregation_query(self) -> aggregation.AggregationQuery:
     def add(
         self,
         document_data: dict,
-        document_id: str = None,
+        document_id: Union[str, None] = None,
         retry: retries.Retry = gapic_v1.method.DEFAULT,
-        timeout: float = None,
+        timeout: Union[float, None] = None,
     ) -> Tuple[Any, Any]:
         """Create a document in the Firestore database with the provided data.
 
@@ -121,9 +121,9 @@ def add(
 
     def list_documents(
         self,
-        page_size: int = None,
+        page_size: Union[int, None] = None,
         retry: retries.Retry = gapic_v1.method.DEFAULT,
-        timeout: float = None,
+        timeout: Union[float, None] = None,
     ) -> Generator[Any, Any, None]:
         """List all subdocuments of the current collection.
 
@@ -156,9 +156,9 @@ def _chunkify(self, chunk_size: int):
 
     def get(
         self,
-        transaction: Transaction = None,
+        transaction: Union[Transaction, None] = None,
         retry: retries.Retry = gapic_v1.method.DEFAULT,
-        timeout: float = None,
+        timeout: Union[float, None] = None,
     ) -> list:
         """Read the documents in this collection.
 
@@ -187,9 +187,9 @@ def get(
 
     def stream(
         self,
-        transaction: Transaction = None,
+        transaction: Union[Transaction, None] = None,
         retry: retries.Retry = gapic_v1.method.DEFAULT,
-        timeout: float = None,
+        timeout: Union[float, None] = None,
     ) -> Generator[document.DocumentSnapshot, Any, None]:
         """Read the documents in this collection.