Fixes #24227.

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Summary by cubic

Stop emitting empty example attributes in Kotlin Spring–generated annotations. Only include example when the schema defines one; removes example = "null" and example = "" from outputs. Fixes #24227.

• Bug Fixes
  • Overrode AbstractKotlinCodegen#toExampleValue to return null unless schema.example is set, omitting default/empty examples in annotations like @Schema/@ApiModelProperty.
  • Updated KotlinSpringServerCodegenTest expectations and regenerated Kotlin Spring samples so annotations now include only description/required when no example is provided.

^{Written for commit e2da581. Summary will update on new commits.}