handle null collation on FTS indexes

[mosheb]

Default FTS indexes have null collation and class throws exception when trying to read as reader.GetString() does not null check.

[mysql-oca-bot]

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[mosheb]

I submitted via email a scanned pdf of the OCA on Tues 19th

[mysql-oca-bot]

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Thanks

[mysql-oca-bot]

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Thanks