fix: mypy checks violation in AzureBlobStorage

[bowenliang123]

Summary

• to fix mypy checks violation in AzureBlobStorage #13216 bug introduced in mypy checks violation in AzureBlobStorage #13216
• to fix the violation in mypy checks:

extensions/storage/azure_blob_storage.py:25: error: Incompatible types in assignment (expression has type "None", variable has type "DefaultAzureCredential")  [assignment]
extensions/storage/azure_blob_storage.py:67: error: Argument "account_url" to "BlobServiceClient" has incompatible type "str | None"; expected "str"  [arg-type]
Found 2 errors in 1 file (checked 952 source files)

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[bowenliang123]

cc @laipz8200 @crazywoola