Allow schema-less UDFs to not be built-in

[roji]

Since #23381, UDFs which have a null schema cause a SqlFunctionExpression with BuiltIn=true to be created. This causes incorrect SQL generation.

(SqlServer isn't affected because of SqlServerDbFunctionConvention, which sets the schema of all non-builtin UDFs with a null schema to dbo)

[ghost]

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[ghost]

Apologies, while this PR appears ready to be merged, I've been configured to only merge when all checks have explicitly passed. The following integrations have not reported any progress on their checks and are blocking auto-merge:

1. Azure Pipelines

These integrations are possibly never going to report a check, and unblocking auto-merge likely requires a human being to update my configuration to exempt these integrations from requiring a passing check.

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