BugFix: An error occurs when the password option is not passed.

[LoveYou3000]

BugFix: An error occurs when the password option is not passed.

For example:
vectordbbench milvusivfflat --case-type Performance1536D50K --uri http://localhost:19530 --lists 1024 --probes 64

An error occurs:
File "pydantic/main.py", line 347, in pydantic.main.BaseModel.init
pydantic.error_wrappers.ValidationError: 1 validation error for MilvusConfig
password
object of type 'NoneType' has no len() (type=type_error)

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: alwayslove2013, LoveYou3000
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