[Bug] Fix the incompatibility of sql mode between Doris and MySQL

[xy720]

Proposed changes

Introduce by pr #4359

VariableMgr.fillValue() method should not call in ExpressionFunctions.eval(), because in method analyzeImpl() of SysVariableDesc, it has been already called once.

If VariableMgr.fillValue() was called twice, the type of SysVariableDesc will become BigInt, which is incorrect.

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• Bugfix (non-breaking change which fixes an issue)

Checklist

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• I have created an issue on (Fix [Bug] Fix the incompatibility of sql mode between Doris and MySQL #7106 ) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If these changes need document changes, I have updated the document
• Any dependent changes have been merged

Further comments

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[morningman]

LGTM

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PR approved by at least one committer and no changes requested.

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PR approved by anyone and no changes requested.