bug: `SELECT WITH(0)` is accepted, but renders as invalid `SELECT WITH(, 0)`

[makenowjust]

Found by fuzzing (see #415).

Repro

stmt, err := memefish.ParseQuery("", "SELECT WITH(0)")
// err == nil
sql := stmt.SQL()
// sql == "SELECT WITH(, 0)"
_, err = memefish.ParseQuery("", sql)
// err != nil: syntax error: :1:13: unexpected token: ,

Cause

parseWithExpr (parser.go:2651) accepts zero name AS expr variables — the lookahead loop can run zero times — but WithExpr.SQL() (ast/sql.go:583) renders "WITH(" + sqlJoin(w.Vars, ", ") + ", " + w.Expr.SQL() + ")", producing a leading comma when Vars is empty.

Per the GoogleSQL grammar (googlesql/parser/googlesql.tm), a WITH expression requires at least one variable:

with_expression {ASTExpression*}:
    KW_WITH_STARTING_WITH_EXPRESSION "(" with_expression_variable_prefix "," expression ")"

(with_expression_variable_prefix is one-or-more identifier AS expression.)

So WITH(0) is not valid GoogleSQL, and the right fix is in the parser: parseWithExpr should require at least one variable. The renderer is then always correct as written.