Makes me wonder if build_to_thrift should return an Option, to make sure callers don't forget to check .valid().

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

@progval pointed out that the current API for building ColumnIndex requires users to remember to call valid()

          Makes me wonder if `build_to_thrift` should return an `Option`, to make sure callers don't forget to check `.valid()`.

Originally posted by @progval in #6042 (comment)

Describe the solution you'd like
Change ColumnIndexBuilder::build_to_thrift so it returns Option - specifically None if valid() is false

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