SnowflakeIO: do not quote empty fields

[turb]

When sending CSV to Snowflake for COPY, do not quote empty fields ('').

With some types, like integer, it leads to an error like this: net.snowflake.client.jdbc.SnowflakeSQLException: Numeric value '' is not recognized.

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[Abacn]

Thanks for the fix. It sounds a valid edge case.

Ideally is there a way to unit test the change? e.g. to confirm

before: "1","","3","",
after "1",,"3",,

(not "1",,"3")

it does not introduce side effect like dropping a field, making the column mismatch

[Abacn]

consider rename this private method (and its overload above) to sth like "quoteNonEmptyField" for clarity

[turb]

fixup 016e641

[turb]

Ideally is there a way to unit test the change? e.g. to confirm

Added in fixup 54ec742