fix tracing align by device error

[dh-cloud]

Description

use sql: tracing select * from root.devices.* align by device; return NullPointerException.

Content1 ...

AlignByDevicePlan path is null, then plan.getPaths().size throw NullPointerException

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This PR has:

• [x] been self-reviewed.
  • [ ] concurrent read
  • [ ] concurrent write
  • [ ] concurrent read and write
• [ ] added documentation for new or modified features or behaviors.
• [ ] added Javadocs for most classes and all non-trivial methods.
• [ ] added or updated version, license, or notice information
• [ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [ ] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.
• [ ] added integration tests.
• [ ] been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

[dh-cloud]

@qiaojialin can you help review?