[BugFix] do not set basic auth header if credentials not provided

[trapped]

Why I'm doing:

While the FE already has this logic, the BE would still send an empty Authorization: Basic header to ElasticSearch even if credentials were not provided; this causes a 403 even if the ES cluster has no AuthN/AuthZ settings. This change allows using StarRocks with ElasticSearch external catalogs even with Basic Auth disabled.

What I'm doing:

Adds a check for empty credentials (both username and password) in the ElasticSearch scan reader, in which case the Authorization: Basic ... header is not set.

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.4
  • 3.3
  • 3.2
  • 3.1
  • 3.0

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