sql/sem/tree: fix tuple pretty print when tuple has 1 element

[tigrato]

When using sql/sem/tree.Tuple to generate SQL statements for IN operators, the produced output is incorrect if the tuple only has one element. The output includes an extra ,.

As an example, the following

&tree.Tuple{
        Exprs: []tree.Expr{
                tree.NewStrVal("val1"),
        },
}

Is incorrectly mapped to ('val1',) instead of ('val1').

Because of the extra ,, sql parsers fail to parse the input.

This commit aims to fix these conditions and remove the trailing ,.

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