Fixing SQL Import Export issue with Managed Identity.

[debalinaroy]

Description

There exists an issue with Import Export using Managed Identity where it asks for SQL Authentication Password even though it is not required. This PR fixes the issue by making the password parameter from the Import Export cmdlet optional

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • [X] General release
  • [ ] Public preview
  • [ ] Private preview
  • [ ] Engineering build
  • [ ] No need for a release

• [X] Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request