fix: panic if condition is nil on WHERE / AND clauses

[Houtmann]

There is a bug when you write a WHERE and or AND clause with the condition nil.
The WHERE clause is not written.

package main

import (
	"fmt"

	"github.com/ulule/loukoum/v3"
	"github.com/ulule/loukoum/v3/stmt"
)

func main() {
	var cursorcond stmt.Expression
	q, args :=
		loukoum.Select("col").
			From("table").
			Where(loukoum.Condition("col").Equal("value")).
			And(cursorcond).
			OrderBy(loukoum.Order("col")).
			Limit(1).
			Query()
	fmt.Println(q, args)
}

SELECT "col" FROM "table" ORDER BY col ASC LIMIT 1 []