[fix](ui) Fix boolean values (true/false) displaying as empty in table cells

[qindongliang]

Boolean values were not displayed in table cells because React does not render boolean values as visible text. This fix converts all cell values to strings using String() to ensure proper display.
Affected components:

• content-result.tsx: Query result table
• data-prev.tsx: Data preview table

Affected Version

• master
• 4.0.2
• Other versions not tested

current bug boolean column show :

fix bug boolean column show :

What problem does this PR solve?

Issue Number: close #xxx
Related PR: #xxx
Problem Summary:
When querying tables containing BOOLEAN type columns in the Doris UI Playground, the values true and false were displayed as empty cells instead of displaying the actual text "true" or "false".
Root Cause: In React/JSX, boolean values (true/false) are not rendered as visible text when used directly as child elements. They need to be explicitly converted to strings.
Solution: Use String() to convert all cell values to strings before rendering, ensuring boolean values display correctly while preserving the existing behavior for other data types.

Release note

Fixed an issue where boolean values (true/false) were displayed as empty in the query result table and data preview table in the Playground UI.

Check List (For Author)

• Test
  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
    • Execute SELECT * FROM table_with_boolean_column in Playground or Click Data Preview on Select Table
    • Verify that boolean values now display as "true" or "false" instead of empty cells
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason
• Behavior changed:
  • No.
  • Yes.
• Does this need documentation?
  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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4. Which code was refactored and why was this part of the code refactored?
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