Correct the behavior change of files.upload API method

Refer to integration_tests/web/test_issue_672.py

Author: seratch

slack_sdk/web/async_client.py

@@ -2905,7 +2905,7 @@ async def files_upload(
             }
         )
         if file:
-            if "filename" not in kwargs and isinstance(file, str):
+            if kwargs.get("filename") is None and isinstance(file, str):
                 # use the local filename if filename is missing
                 if kwargs.get("filename") is None:
                     kwargs["filename"] = file.split(os.path.sep)[-1]

slack_sdk/web/client.py

@@ -2848,7 +2848,7 @@ def files_upload(
             }
         )
         if file:
-            if "filename" not in kwargs and isinstance(file, str):
+            if kwargs.get("filename") is None and isinstance(file, str):
                 # use the local filename if filename is missing
                 if kwargs.get("filename") is None:
                     kwargs["filename"] = file.split(os.path.sep)[-1]

slack_sdk/web/legacy_client.py

@@ -2859,7 +2859,7 @@ def files_upload(
             }
         )
         if file:
-            if "filename" not in kwargs and isinstance(file, str):
+            if kwargs.get("filename") is None and isinstance(file, str):
                 # use the local filename if filename is missing
                 if kwargs.get("filename") is None:
                     kwargs["filename"] = file.split(os.path.sep)[-1]