Change default table_types to None

With UC, `table_types=[]` is interpreted differently than on HMS. On HMS, it means no filter applied, while on UC it means a filter of nothing applied, i.e. on UC we get the following:
```
cursor.tables(schema_name="taxi")
2>>> []
3cursor.tables(schema_name="taxi", table_types=["TABLE", "VIEW"])
4>>> [Row(TABLE_CAT='shant', TABLE_SCHEM='taxi', TABLE_NAME='trip_view', TABLE_TYPE='VIEW', REMARKS='', TYPE_CAT=None, TYPE_SCHEM=None, TYPE_NAME=None, SELF_REFERENCING_COL_NAME=None, REF_GENERATION=None), Row(TABLE_CAT='shant', TABLE_SCHEM='taxi', TABLE_NAME='trips', TABLE_TYPE='TABLE', REMARKS='', TYPE_CAT=None, TYPE_SCHEM=None, TYPE_NAME=None, SELF_REFERENCING_COL_NAME=None, REF_GENERATION=None)]
```
Change `table_types` to default to None to get all the table types by default.

Author: NiallEgan

cmdexec/clients/python/src/databricks/sql/client.py

@@ -295,8 +295,6 @@ def tables(self,
         Names can contain % wildcards.
         :returns self
         """
-        if table_types is None:
-            table_types = []
         self._check_not_closed()
         self._close_and_clear_active_result_set()
 

cmdexec/clients/python/src/databricks/sql/thrift_backend.py

@@ -653,8 +653,6 @@ def get_tables(self,
                    schema_name=None,
                    table_name=None,
                    table_types=None):
-        if table_types is None:
-            table_types = []
         assert (session_handle is not None)
 
         req = ttypes.TGetTablesReq(