[ci] actually fix nullness in sql config (#8500)
[ci] actually fix nullness in sql config (#8500)

I fixed sql-config.cnf in the last change, not sql-config.json.

I just changed everything to treat a key being `null` in JSON the
same as the key not existing.

The situation now:
- sql-config.cnf will not have the string None (seems right, cnf is for MySQL)
- sql-config.json might have `null` which is treated the same as a missing key
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